Sinking ships before reaching patrol zone

Hi there.


Just a little historical question about permissions and orders from BDU.
Sometimes, it was prohibited for kaleuns to sunk lonely merchants (I'm not talkin' about convoys) before reaching their proper patrol zone or missions area. The main goal being to remain discreet and undetected until the designated patrol grids and save the torpedoes, like Paukenschlag for instance.

Was it a mandatory rule from BDU or an order used sometimes for specific operations and patrols ?

Saw a lot of players who don't hesitate to engage lonely merchants ships while "en route" to their patrol zone and I wanted to know how this practice was tolerated by BDU historically.

Thanks in advance.