R12 - GL / XLA / FAH - Reporting Currencies

There are three levels of reporting currencies.
Subledger, Journal and Balance.

The data will be create for those ledgers, when the following processes have been run.
Create Accounting - Journals will be created in SLA.
Transfer to GL & Journal Import - Journals will be created in GL.
Post to GL - Balances will be created / updated in GL.

Subledger Level - It maintains data in SLA for ALC also, you can view the accounting in SLA OA pages and other reports. The SLA tables like xla_ae_headers, xla_ae_lines and xla_distribution_links and GL tables like gl_je_batches, gl_je_headers, gl_je_lines and gl_balances will have data.
Journal Level - Data won't be present in SLA for this ALC and GL will have. Only GL tables like gl_je_batches, gl_je_headers, gl_je_lines and gl_balances will have data. SLA tables won't.
Balance Level - Data won't be present in SLA and GL journals. Only Balances will get updated / created for this ALC. gl_balances will have data.

Balance level, as they just have balances, drilling down to the GL journal entry can't be happen.
Journal Level, we can see what are the journal entries for that ALC, which contributes to the balances.
Subledger level is more detailed.
Primary Ledger Accounting happens in SLA and it maintains the link between subledger data to SLA. And that can be transferred to GL either in Summary / Detail.
As Journal level only replicates the GL journals into the secondary ledger, you might not know which subledger transaction has contributed to this.
Subledger level maintains the link between subledger data to SLA and GL. So, drilldown can happen till subledger data.

Ex: Payables - Invoice
Subledger level -> GL to Invoice link will be there through SLA
Journal level -> GL journal will be there, you might not know which invoice(s) contributed to this.
Balance level -> ONLY GL Balances will be maintained, you might not know, which GL Journal have contributed to this.