If your orders said something to the effect of "Deploy to
the area of XX and engage enemy merchant shipping" (with "the area of XX" being any one of a number of locations), then you have a "Sink" mission objective. When you arrived at the "Star" location, did you get a message "ENGAGE ENEMY MERCHANT SHIPPING IN AREA XX"? That would be the 'Part 2' of the mission. You generally have to sink 10,000 tons worth of shipping. But it doesn't come to you, you have to go find it. You may have to sink two or three ships, depending upon their sizes.
If your orders said something like "Patrol
area XX and engage enemy shipping" (with "area XX" being any one of a number of locations), then you have to stay within a given distance of the "Star" for a given time period. Generally speaking, you are talking about a 'radius' of 100 kilometers, or a diameter of just over 100 nautical miles. Draw yourself a circle that size (I use 75nm to be 'safe'), and stay within it. The time period is usually 2880 minutes, which is 48 hours, or two days...
There are also SpecOps that you could be assigned to, such as a photo recon, or an insertion of a spy or coast watcher or supplies, or maybe they want you to lifeguard for downed pilots. These missions have unusual circumstances. If you have one, let us know.
If you have a mod, all of the above can be and probably are changed.
