Layout Width and Height - PX Exact Size

Would anyone be able to explain why the Layout Width and Height, PX Exact Size setting includes options for Theme, % and Formula when these options are always shown as Unsupported Binding?

I appreciate the option is labelled PX Exact Size, hence I guess that means it’s expecting a px entry but if so, why are the other options shown at all.

Are there any circumstances where Theme, % and Formula can actually be specified under PX Exact size and if so when would that be?

Many thanks

Unsupported binding