but they never expand on what the 'modification' is.

Well, in defense on this; modding your forum's stock software (the UBB) does invalidate your license terms for support according to the license

... Plus, UBBDev is technically the place for mods :x...
In that case what type of changes are acceptable, and how do you define 'stock' software

and what type of changes would then invalidate ones license terms for support
P.S: also am i correct in understanding that if modifications are done by UBBDev then the license agreement would not be invalidated....