In the standard alphabetic scheme, you can add the following property to identify the characters that should never be used in revision labels:

where <list> is the list of characters to exclude. Do not use a separator in the list.

By adding this property, the characters in the list cannot be used in either existing or new revision labels. For example, assume the following property is set:

Then valid revision labels will be A-H, J-W, Y, Z, AA-AH, AJ-AW, AY, AZ, (and so on).