Ledger Security in R12 Req: To show only 1 ledger to the responsibility.

Profile:

'SLA: Enable Data Access Set Security in Sub ledger'

If you have more ledger and you want to restriction no one show the other you can choose this profile option and make it yes value in site

If you have primary and secondary please check this note

The primary and secondary ledgers displayed in the ledger list of values are determined by the system profile option SLA: Enable Data Access Security in Subledgers. If this profile option is set to Yes, the list of values displays only the primary and secondary ledgers defined in the GL: Data Access Set and the SLA: SLA : Additional Data Access Set profile options. If the profile option is set to No, the list of values displays all primary and secondary ledgers.

And  check this url

https://docs.oracle.com/cd/E18727_01/doc.121/e13628/T149412T282264.htm