What “National Identification Number” to enter in an ESTA application, for a German?

I’m trying to complete an ESTA application (German citizen). One of the questions asked is about a National Identification Number, with this explanation:

Enter the number on your identity document, other than a passport, issued by your country. The National Identification Number, if issued to you, is required to complete the application. If you do not have a National Identification Number, enter UNKNOWN.

This leaves me wondering what I should enter. “Number on your identity document, other than a passport,” sounds like they are asking for the identity card number (Personalausweis). The term “identification number”, on the other hand, rather makes it appear like a permanent identifier such as the tax payer identification number (Steuer-Identifikationsnummer). It’s also conceivable that neither is applicable and I should enter unknown instead.

I’ve tried searching for further information on the embassy’s website, but found this number mentioned in one (unhelpful) place only; apparently the question was added to the form less than a year ago, which might explain why I only find few mentions elsewhere on the web, and they are all inconclusive.

Interestingly, this question appears only after I’ve selected the country of citizenship (Germany in this case), and it does not appear for many other countries; e.g., this question is asked for citizens of the Netherlands or South Korea, but not of Austria, Switzerland, Sweden or the UK.