In my own experience aboard ship, relative bearings are only used internally to the spotting ship and true bearings are used in communications between ships.
Relative bearings are only meaningful to individual ships because only that ship knows its true heading. What happens is, the lookout sees something and passes the word to the bridge using relative bearings, so the skipper/OD/quartermaster/whomever can immediately look in that direction to see it for themselves without having to run over to check the pelorus first.
Then, when the ship sends a contact report to a distant fleet commander, she gives her own position and then the true bearing to the target. That way, the distant commander, who can't see either the spotting ship or the target, can mark them both on his chart.
