Category
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.