Off hand I'd suggest creating a new relationship that filters out the undesired records. How exactly to do that requires a little additional information.
I see two tables (or table occurrences) in your expression:
qt_QTITM and qt_PRCH
What does the relationship linking these two tables look like?
kf_QuoteID = kp_PurchasingID is creating another instance like this that includes Category_C = PURCHASE ORDER the only way? I figured there would be some way to script it, but I am relatively new to scripting
I find your description rather confusing. To take a simple example using Parent and Child tables:
IsEmpty ( FilterValues ( "PURCHASE ORDER" ; List ( Child::Category ) ) )
returns true when NONE of the related child records is in the PURCHASE ORDER category (when evaluated from the point-of-view of Parent).
thanks guys, I decided to just create another relationship