When working with databases, each row within any table requires a Unique Identifier that's UNIQUE to indenify the ROW. In QODBC, that Unique Identifier is ListID for List and Transaction Header type tables and TxnLineID for Line Transaction Type Tables.

The FQPrimaryKey is used by QODBC internally to indentify each transaction line with it's transaction header row and is made up of the TxnID and TxnLineID strings combined with a "|" for each line in a Invoice, Sales Order, Bill line etc. like this:-

While you can find by FQPrimaryKey like this:-

you normally use TxnLineID when working with QODBC like this:-