Re-enter Schengen after 6 months for 90 days [duplicate]

My mother has a multiple-entry visa issued for Belgium which is valid for one year, with maximum stay of 90 days.

Having entered the Schengen area June 25, her 90 days expires September 23 (she will exit September 20).

Since she has used up her 90 days, how would one go about restoring another 90 days when the 180 days are up (December 22)?

Does she need to reapply for a new visa (despite the fact that her current visa has not expired)?

Or does the 90 day limit reset automatically and she can re-enter with her existing visa?

Best Answer

I found the answer to my question here: Multiple entry Schengen Visa valid for 5 years, with duration of stay : 90 days

The answer makes clear reference to the Schengen Code in which the short version is: If a multiple entry visa is granted for X number of years and the maximum duration of stay of 90 days is granted, the number of days indicated in duration of stay is for any six-month period, not the visa period.