How do unsold first-class seats get used?

This details are specific to each airline and generally fall into two categories -- fill all the first class seats or let them fly empty.

The airlines that want to fill first class will have upgrade programs in place. These can be automatic upgrades for certain classes of frequent fliers down to paid upgrades from economy class.

Within an airline, this policy can also vary for the type of service. The case I'm most familiar with is Continental Airlines (no longer an entity, merged with United). For domestic service the first class seats almost never went empty. They would go to lengths to upgrade anyone they could into those seats at the gate prior to departure. Any that happened to be empty would then be filled by non-revenue travelers (employee deadhead, jumpseats, etc) but only after economy was filled first. This could, on occasion, result in empty first class seats, but In my experience that was rare. In this case first class was generally used by people who paid full fare and any eligible frequent fliers from the economy cabin.

For international travel on Continental they did not upgrade into their "businessfirst" class at the gate. This was great for non-revenue travel because while you could generally never count on a first class seat getting to a hub, you could generally get one for the long intercontinental legs. In this case the first class seating got used by people that paid full fare, elite frequent flier upgrades and non-revenue travelers.

Other airlines policies will vary but generally fall somewhere in the spectrum between the two cases above (that come from just one airline).

In your case of 8 unsold first class seats they will probably stay that way until a few days before the flight unless people spend the money to buy them earlier. A few days before the flight the airline may start upgrading certain frequent fliers into first class. Certainly by the day of the flight they will have upgraded frequent fliers if it their policy to do so. Starting at check-in time (generally 24 hours prior to departure), they may start offering paid upgrades from economy, if it is their policy to do so. In the gate area prior to boarding they'll make their final upgrades (frequent flier and paid, as the situation warrants). At 15 minutes prior to pushback they will finalize standby (revenue standby and non-revenue travel) passengers and may (or may not) use the employee travelers to fill any remaining seats. Finally, they may upgrade the lowest of the low non-revenue classes, the buddy pass riders and inter-line travel (e.g. ID90). All of the above are optional and the airline may simply let first class fly empty rather than provide unpaid (or reduced fee) upgrades into the first class cabin.

Keep in mind that U.S. airlines generally differentiate between domestic and international flights, with upgrades being much easier on domestic flights. Most airlines have various programs to allow passengers to upgrade into empty premium seats, and typically, elite members of the frequent flyer program are given free or low-cost upgrades in advance of a domestic flight. Most airlines also allow frequent flyer members to spend miles for an upgrade, for both domestic and international flights. Some airlines sell empty seats at a low cost 24 hours before a flight, first-come first-served, but this is usually for domestic flights. Some airlines sell upgrades for all flights at check-in time or at the airport if they anticipate having lots of empty seats. These upgrades typically can cost anywhere from $50 to over a thousand dollars.

Separately from all these "normal" upgrades is what's known as "operational" upgrades. This is when the airline has oversold the flight. Typically, coach is oversold but seats may be available in business and/or first. So, the airline will "roll" the cabins: they've move people from business to first (if there is a first class), from premium economy to business (if there is premium economy), and economy to premium economy (if it exists) or business. This is done at the gate or even after passengers have boarded. This is what happened when you hear people say "I got a free upgrade" or "I dressed in a suit and I got upgraded!" The airline needed to upgrade some people to fit more people on the flight.

Length of travel is also a concern, when I worked for the big airline the domestic empty first class generally gets fill with people with high frequent flier status.

When it comes to international travel (think 6+ hour flights) they would rather fly empty in first class than giving out cheap upgrade options. Considering that a first class ticket is around 10 times the price of economy, they do not want to risk devaluing their first class, as well as a way to keep the first class passengers keep buying the seats.

As others have mentioned, most U.S. airlines that operate multi-class aircraft have a system for upgrading frequent fliers with their airline into first class seats for free. To provide a concrete example, here's how Delta's system works:

Complimentary upgrades for elites are only allowed on flights that do not offer Delta's "DeltaOne" long-haul business class product. "DeltaOne" basically means anything with the lie-flat beds, unless those aircraft are being operated on short-haul routes for repositioning purposes or some such thing.

The system is entirely automated, so upgrades are processed by the computer rather than by the gate agent, removing the incentive to just throw whoever checked in last in the upgraded seat.

• Any Medallion member who books a Y-class fare (full-fare economy) ticket is upgraded to First immediately at ticket purchase.

• 5 days before check-in opens for the flight (i.e. 6 days before departure,) anyone with Diamond or Platinum Medallion status will have their reservation automatically upgraded to first class.

• 3 days before check-in opens, Gold Medallions will be upgraded to First.

• 1 day before check-in opens, Silver Medallions will be upgraded to First.

However, only a certain number of First-class seats will be open for upgrade at a given time. If there are not enough open seats in First to upgrade everyone who is eligible at a given time (which is almost always the case,) upgrade priority is as follows:

1. Y fare class.

2. Highest status tier.

3. If status tier is equal, highest fare class purchased.

4. If both status tier and fare class are equal, whoever bought their ticket first.

If not all Medallion members have confirmed upgrades by the time of flight check-in, the remaining Medallion members will be ranked according to the above priority and, as seats are released for upgrade availability, they will have their upgrades confirmed in that order. The last few First class seats are usually released shortly before departure. Monitors in the gate area display the current upgrade list and Medallion members can also access it either on Delta's website or their mobile app after they check-in.

Note that the above process applies only for complimentary upgrades to First Class. Complimentary upgrades to their "Comfort+" (premium economy) product are available at time of ticket purchase for Diamond and Platinum Medallion members, 72 hours before departure for Gold Medallion, and 24 hours before departure for Silver Medallion. These must be selected manually by the passenger. Complimentary upgrades to C+ are available for on all Delta flights that have C+ seats open, regardless of distance.

While this is all specific to Delta (because that's the one I'm most familiar with,) American, United, and Alaska Airlines have similar systems for their elite members.

Airlines will "bump up" passengers sometimes, either by automation or by request. It depends on the airline and even the person you are checking in with at the desk.

My anecdote:

My family were going on vacation about 20 years ago. I was very young, in the region of 8 years old. Travelling with my father, mother and brother. The tickets would have been a variation of "Economy".

We had finished check in, and were getting ready to leave the desk. My brother turned around and went back to the desk. Somehow, he got us all bumped up to First Class. To this day, we do not know what he said, and neither does he (or so he claims).

American Airlines is notorious for sending out first class seats empty before allowing them to be redeemed for awards. I had a first class award through Chicago to Bangkok, and the flight from Charlotte to Chicago only had economy award seats, so I flew economy on one segment and first on the other three. When the plane boarded there were six seats open in first class and they would not allow us to sit up there, even though we were on a first class upgrade itinerary. They were unyielding and gave us absolutely no rationale for this.