Hi,
If you base a rule on WBS, then it's indeed stored in internal format and in this format appears in COBL structure. So, if you want to take part of WBS's string, you'll need first to do a conversion. By doing so, you can populate a user-field USERTEMP1 and base your rule of type 'Table Lookup' on it. Alternatively, if you want more flexibility, create a rule of 'enhancement' type: in this way you will be able to program whatever you want. It will create a user-exit in SAPLFMDT include...
Regards,
Eli