P4310 - budget checking

in P4310 detail screen the extended cost is composed of AEXP+ACHG (foreign FEA+FCHG); some PO's that user's have created have a value stored to ACHG instead of AEXP.  From the knowledge base, ACHG is  used to store the "Amount on Hold" - which seems to indicated that there is checking against a budget being performed.  It appears that a Processing Option value controls whether "budget checking is active"  -- which Proc Opt is it?