[Trevally.]

When you get a mission say - sink 50,000 tonnes,
you do not have to sink it all in one patrol. You can use
as may patrols as you like.

If you sink 10,000 in the patrol, you can return to base - end patrol
and select it again when you are ready. Your 10,000 tonnes are saved.

You can even try a different mission and go back to it later. (you dont even need to select it again.)
You only have to get the 50,000 tonnes by the end of the date given
from the area stated.