global activity codes sort out-of-order

 2 part question. both about global activity codes.

Part A - are there reserved characteres that should not be used in global activity code values?  Specifically, is the period "." reserved.

Part B - when sorting two sort levels, based on two different global activity codes - no grouping - the results are not sorting in order.  For example:  sorting by priority, then location ...ascending for each.  /the second sort appears to be reverting back to the order entered.  But, even the first level sort doesn't sort priority correctly.  Pattern / format for the priority = 1-00, 1-01, 1-02, etc.

Ideas?