Crosses are (strategic) victory points. Complete the "campaign goal" (for example in your pic British Coastal Waters is a completed goal, while British Supplies is not) and you get those victory points.
Doesn't matter how many patrols you complete. You're still roaming freely, may meet a ship outside of your current "mission", sink it, and find out it completes something for the "campaign".
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With strength I burn...
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