AFAIK: Laszio is correct
Expressions are not used in components unless they are set to custom value which would be the same as using a property. The Classification still must be set properly
You can assign properties to Building Materials and. assuming the classifications are set correctly, they will show up in the Components schedule only if that element has that BM. BM's, also, can not use expressions.
A extremely limited feature that ,hopefully, will be expanded in the future?
Windows 10 - Visual Studio 2019; ArchiCAD 26