Yes. Exactly. I had Request Assign. My confusion stems from the documentation. This is what it says about 'Request_Assign':

"If you find a record manually (with the Find command) and want to have the DDO “latch onto” and display the current record, you can send the Request_Assign message. Request_Assign will cause the DDO and its parent DDOs to accept the record(s) in the buffer as their current record and refresh all affected DEOs."

But the documentation on Request_Relate does seem more appropriate in this case of Relate_Main_File:

"If you are going to find an additional record in Relate_Main_File, you must notify the DDO that you have found this new record. You do this by sending the Request_Relate message (if the record was found) or Request_Clear_File (if the record was not found). Request_Relate will also perform a relate on the newly found record."

Thanks again for clearing this up.