One method is to use a value list of only the permitted letters.
You can also use a validation rule that only accepts specific letters.
Length ( Filter ( self ; "ABCDEF" ) ) = 1
Or an auto-enter calculation can simply reject all letters that aren't accepted:
Left ( Filter ( self ; "ABCDEF" ) ; 1 )
(Expressions also limit input to a single letter.)
That was exactly what i was looking for.
Thanks a lot!