Cross Validation Rules: Overview
In Oracle Fusion General Ledger, use cross validation rules to determine the account combinations that are created dynamically as your users enter transactions or journal entries. Once enabled, a cross validation rule determines whether a selected value for a particular segment of the account combination can be combined with specific values in the other segments to form a new account combination.
A cross validation rule is defined in terms of a condition filter and a validation filter.
The condition filter describes the event under which the rule will be evaluated. If the event specified in the condition filter is not applicable, then the rule will not be evaluated even if it is enabled.
When the event specified in the condition filter is applicable, the validation filter condition must be satisfied before the account combination can be created. The rule is evaluated using the following logic:
If condition is satisfied, then perform specified validations.
For example, if your organization has determined that a certain company value, Operations, cannot use a specific cost center, Marketing, define the following cross validation rule to validate your accounts accordingly: If company is equal to Operations, then validate that cost center is not equal to Marketing.
If account combinations already exist and violate the newly enabled cross validation rules, these account combinations continue to be valid. Before disabling any existing account combinations that violate your rules and you are no longer using, move the balances in those accounts to the correct accounts. Then disable the account combinations manually to prevent further posting.
https://docs.oracle.com/cd/E28271_01/fusionapps.1111/e20375/F350915AN2814A.htm

Data Conversion Levels for Reporting Currency Ledgers:
https://docs.oracle.com/en/cloud/saas/financials/20b/faigl/ledgers.html#FAIGL300044 Reporting currencies have the following three conversion levels:
Subledger level: Use SLA and the GL Posting programs. This level is equivalent to full MRC. A subledger level RC can only be assigned to primary ledgers.
Journal level: The GL journals only are replicated via the Posting program. The journal created for the RC is created within the same batch of the primary ledger journal batch and is automatically posted. This level is the equivalent of thin MRC.
Balances level: Use the GL Translation program to maintain balances in the reporting currency.
Note: Journals can be entered directly into the subledger and journal level reporting currencies.
Ref: Note 462088.1 R12: Accounting Setup Manager Concepts