Sub-groups in Touchworks are found in TWAdmin under the Group Admin VTB What defines a sub-group is not a simple answer. Sub-groups can be whatever they need to be. They are entities that have something in common with regard to the Touchworks Charge module. They are often the sub-categories on the former paper charge slip.
If we defined a group as OBGYN. We would build sub-groups for diagnoses, procedures, modifiers, etc. for OBGYN. As we are building the sub-groups they are assigned to a group. For example we might create a sub-group for "new patient visit charges" and additional sub-groups for "established patient visit charges" and "Consult visit charges" etc.