Accounting Attributes: When creating a journal line rule, accounting attribute assignments are automatically established. These are based on the default accounting attribute assignments for that journal line rule's accounting event class. You can override this default mapping of standard sources to accounting attributes. The list of values for the source override includes all sources assigned to the accounting attribute, for the event class associated with the journal line rule.
https://docs.oracle.com/en/cloud/saas/financials/21a/faisl/subledger-accounting- setup.html#FAISL216141 Accounting Attribute Assignments: Points to Consider The Create Accounting process uses the values of sources assigned to accounting attributes plus accounting rules to create subledger journal entries. Almost all accounting attributes have sources assigned at the accounting event class level. Depending on the accounting attribute, the accounting attribute assignment defaulted from the accounting event class can be overridden on journal line rules or subledger journal entry rule sets.
Once sources are assigned to accounting event classes, they are eligible for assignment to accounting attributes for the same accounting event classes.
The Create Accounting process uses these assignments to copy values from transaction objects to subledger journal entries. For example, you may map the invoice entered currency to the subledger journal entry entered currency.
https://docs.oracle.com/cd/E25178_01/fusionapps.1111/e20375/F644879AN57324.htm