R12 General Ledger - Subledger Accounting Option on Secondary Ledger for Fixed Assets

Fixed Assets question but related directly to GL system architecture:

Primary Ledger with a Secondary Ledger tied to it, with a conversion method of Subledger. COA etc are the same but that is not particularly relevant to my question.

Navigation: Setup > Financials > Accounting Setup Manager > Accounting Setups > Query the Secondary Ledger > Under 'Subledger Accounting Options' = click 'Update'. Under 'Assets' click 'Update Subledger Accounting Options'. There is a flag called 'Use Primary Ledger Amounts'.

What specifically does this 'Use Primary Ledger Amounts' flag do? It is a flag that is unique to Assets in the SLA Options. Other Subledger modules (such as AP or AR) do not have it. So what exactly does it do from a SLA conversion and posting perspective?