What purpose does Choosing Ledger in Receivables Option serve

When we define a ledger, we associate secondary ledgers to it.

In the receivables responsibility> System Options> We first choose the Operating Unit, then we are asked to Choose the Name (Set of Books)  where the Ledger Names are available, both primary and secondary ledgers are there.

Because the setup and relationship is already defined in the Accounting Manager page, what benefit

The issue has come for me because when defining the system option I have chosen the incorrect  Ledger, I chose the secondary Ledger Instead of Primary ledger. However for the responsibility there is a Ledger already assigned. So which would take preference while doing the transactions?