XS_VALID_CHARS_FOR_ASSEMBLY_FAMILY_QUALIFIER
Category in Advanced options dialog box
Numbering
Specifies valid letters for the assembly family number qualifier. 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 it 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.
Example
XS_VALID_CHARS_FOR_ASSEMBLY_FAMILY_QUALIFIER=GHJKL