A chase or isolated action could place a sixth rate against a larger ship when no other warship was available. That circumstance did not make the pairing sensible in a fleet battle. Fifth and sixth rates had strong reasons to keep clear of third rates when larger line-of-battle ships were present. The distinction makes ship ratings useful as guides to tactical employment as well as size and prestige.
Notable Point ยท 12:52-13:16
Why smaller ships avoided the battle line
A chase could force a sixth rate to face the only enemy available, but fifth and sixth rates normally kept clear of third rates in a fleet action.