if my memory serves me correctly from various self inflicted history lessons BDU would send out several boats into an area.
For Example:
U-46 would be issued orders to proceed to grid AM54.
U-90 and U-121 would be issued orders to proceed to grid AM52, one boat would cover the north side of the grid and one the south
U-88 and U-76 would be issued orders to proceed to grid AM02, and again one boat would cover the north side of the grid and one the south.
or some similar setup
when one boat made contact with a convoy, he would promptly report it for example:
"U-76, sighted convoy in grid square AM02 Westbound, approximate speed 4 knots, sighted approximately 4 escorts, maybe more, at least 30 merchants in the lanes. advise."
BDU would respond with something like
"U-76 follow convoy but do not attack until other boats are on station, broadcast beacon signals to aide in the navigation of other U-boats. provide regular updates as well."
then, over the course of the next few days, the other U-boats would close in on the sighted convoy and surround it if possible, when all boats reported ready the attack would begin. The idea was that a convoy surrounded by 4 to 6 U-boats would be more or less obliterated, and many times in the north atlantic from 1939 to early 1943 thats exactly how it worked.
Other boats like an IXB would sometimes receive orders differently... a larger ocean going long range boat would receive orders like "Proceed to the United States Eastern Seaboard and patrol an area extending from 150 nautical miles off the coast of New York, to an area off the coast of Cape Fear and attack targets of opportunity with particular attention paid to tankers"
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