it says that it's down to the individual oath taken by that individual Marine. Often it is for "one mission", but the meaning of "one mission" could mean one Crusade, one area of space for a set length of time... you get the drift. It's basically entirely down to the Marine in question and where he is being stationed.
How does the "recruiting" work in general? Does the chapter grant Marines to the Deathwatch on a hunch, does the Deathwatch ask for Marines from specific chapters? And who exactly is "The Deathwatch" anyway? How are they organized?
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