Legal Entities and Primary and Secondary Ledgers
The best practice is to only have legal entities of the same country within one primary ledger, even if the ledger currency is common among multiple legal entities.
A ledger currency could be common among multiple legal entities in the example where where legal entities in countries of the European Union share EUR as a common currency.
Legal entities from different countries are likely to have different reporting and regulatory needs. If they're combined into the same primary ledger, it makes it impossible to apply the differing or conflicting accounting configuration requirements of the different legal entities all within the same primary ledger.
Legal Entities and Secondary Ledgers
The legal entities associated with a secondary ledger are strictly and completely derived from its primary ledger. As such, any balancing segment value to legal entity assignment as defined in the primary ledger can be accounted for in the balancing segment value to legal entity assignment in its secondary ledger as well.
You can assign a secondary ledger only a subset of the primary balancing segment values that are assigned to a primary ledger and its legal entities. See the Legal Entity-Specific Secondary Ledgers with Controlled Replication from Primary Ledgers topic for more information on this feature.
If the primary and secondary ledgers share the same primary balancing segment value set, then the primary ledger balancing segment value to legal entity mapping is automatically replicated to the secondary ledger. You can't make any update to this replicated setup in the secondary ledger to ensure this stays in sync with the primary ledger.
If the primary and secondary ledgers don't share the same primary balancing segment value set, then for the secondary ledger, you can separately assign the equivalent primary balancing segment values to the legal entities derived from the primary ledger using the secondary ledger primary balancing segment value set.