It took a while, but I figured out the syntax for combining IIF and DLookup together as a control source for a form.
=IIf(DLookUp("[tblCustody]![Client]","tblCustody","[tblCustody]![LabID] ='" & [txtLabID] & "'")=...
We run an environmental testing Lab. We get water samples directly from cities and towns, or we recieve samples from other labs that subcontract the work to us.
When a city of town send us water directly - they are what we consider the primary client.
When another lab sends us the sample, they...