Explanation:

Box 1: Operating units
Currency, if the organization is modeled as an operating unit.
If your organizations can use a single functional currency, you can model the organizations as operating units.
Operating units must share a functional currency. However, you can enter transactions and create reports in multiple currencies.
Box 2: Legal entities
Currency, if the organization is modeled as a legal entity.
If your organizations must use different functional currencies, you must model the organizations as legal entities. Functional currencies are set up per legal entity. However, you can enter transactions in multiple currencies.
Reference:
https://learn.microsoft.com/en-us/dynamics365/fin-ops-core/fin-ops/organization-administration/plan-organizatio