I have recently found out that if using signing limits with the purchase requisition workflow, that workflow notification placeholders reference the system currency, ignoring both the default currencies on both the vendor and legal entity.

One could argue for either the vendor currency or the legal entity accounting currency, but either would be more appropriate that using the system currency, particularly for companies operating in multiple countries. If there is an argument for using the system currency, it could possibly be based on a legal entity-specific parameter.