Why is "AA" (Lower Rate) not allowed for German reduced VAT ("gemäßigte USt.", 7%) in VAT Category Code BTs?

[lagset]

I hope this repo is the right one to ask this question.

My research based on the XRechnung 3.0.2 specification document, your gitlab repo's discussion and further sources didn't get me anywhere.

So in detail: For BT-95, BT-102, BT-151, BT-118 only the codes S, Z, E, AE, K, G, O, L and M of the code list UNTDID 5305 are valid, according to your specification and validators.

However, in Germany, there exist two VAT rates 19% (Standard Rate, thus Code S) and 7% (Lower Rate).

The intuitive fit for the 7% percent reduced tax rate would be Code AA which is specified as "Lower Rate" and described as "Tax rate is lower than standard rate" in the UNTDID 5305 code list.

Why is it that code S needs to be used for both the 7% tax rate as well as the 19% tax rate?

Thanks in advance!