I believed a circle icon means it is a convoy reported and being shadowed/attacked by another U-boat, nut now I understand that it could also be an AI *enemy* sub being part of the convoy. A bit confusing imo.
Often you will see a smaller circle icon following it; this is the german uboat then.
(By drawing a line between the convoy and the u-boat you can get the convoy's track quite accurately and set an ambush
)