Create a New Operating Unit

I have 2 primary Ledgers.  My USD ledger has 2 operating units.  My CAD primary ledger has 1 operating unit due to the new currncy we just started to use.  We are also moving into Puerto Rico which operates in USD and uses US GAAP.  As such I will record accounting in existing USD primary ledger.  Do I need to create a new operating unit or just anew bank owned by the PR legal entitiy?  What determines when you need to create a new operation unit other than sub ledger that will be assoicated with a new primary ledger?

Thanks