XS_VALID_CHARS_FOR_ASSEMBLY_FAMILY_POSITION_NUMBERS
Category in Advanced options dialog: Numbering
Use this advanced option to specify valid letters for assembly family position numbers. You must specify all the valid letters in this advanced option. By default, the letters A- Z are valid.
For example, you might not want to use D, because it is easy to mix up with O and 0. In this case, you would enter letters A - Z but leave out D.
This advanced option is model specific and the setting is saved in the options database. Reopen the model to activate the new value.