Once you have more than one fiscal entity, every new invoice asks which entity issues it. Three ways to pre-select.
By customer default
On each customer record, set "Default issuing entity". US customers issue from your US entity automatically; MX customers from your MX entity. Override per invoice when needed.
By contract
If the customer record points to a master contract, the contract pins the entity. Useful for international groups where the parent is in one country but invoicing happens in another.
By currency
If you only have one entity that supports the customer's currency, the system picks it without asking. Good guardrail against issuing a USD invoice from your MX entity by accident.
Reporting
Dashboards roll up either by entity (compliance view) or across entities (executive view). Both are first-class.