XS_VALID_CHARS_FOR_PART_MULTI_NUMBERS
Category in Advanced options dialog: Numbering
Use to specify the valid letters for part multinumbers. You must specify all the valid letters in this advanced option. By default, letters a - z are valid.
This advanced option is model specific and the setting is saved in the options database.
Example
XS_VALID_CHARS_FOR_PART_MULTI_NUMBERS=abeg