Also for enterprise systems you need to test concurrency and ensure multiple people can use the system without affecting performance.
Other items to consider would be:
- Connectivity (What happens if many users hit the server at once)
- Simultaneous access testing (What happens if the database gets too large)
In your validation the PQ is the place where, as applicable, confirmation is made that the system properly handles stress conditions applicable to the intended use.
PQ for Performance, OQ for Operational Testing.