I hate summer on the North Atlantic......

[John Pancoast]

propbeanie covered it well. H.sie's patch adds subs for a pack if desired and one can also add them onto convoys themselves in the campaign_rnd.mis file ala NYGM.
The difference between the two methods is:
- with H.sie's patch, you provide an initial convoy contact report upon sighting the convoy. Then you either get a reply to keep providing (hopefully and preferably) hourly contact reports to "assemble the pack" for an attack. After sunset, you get a message stating the pack is ready, attack at will and they eventually start attacking the convoy.
Or you get a reply stating to the effect that the pack isn't available, attack on your own if you want. This type of reply is usually the first one given to to you but it can happen at any time after too.
- with the "add the sub(s) to a convoy" method, there are no contact reports, etc. involved.
They spawn and attack once you get within the game's 3d bubble (30kms or so ?)
- you can also have both at the same time on a convoy; a pack you've called in and one added to the convoy per above.

All the methods can really decimate a convoy sometimes.
H.sie's patch is mostly for the North Atlantic so some locations where pack attacks were prevelant historically don't ever get them via his patch.
For those I've added to the convoys per above. I also include the convoys covered by the "add sub(s)" method in NYGM in my non-NYGM installs. For me, otherwise there is still to many instances of the "you against the world", always attacking alone aspect per the game without these modifications.
Historically, for most of the war in the North Atlantic (including west of the Iberian peninsula area) most attacks should be with a pack vs. alone so I prefer to reflect that as much as possible.
But you have to be careful; as said, for various game reasons convoys can be decimated much more than the historical results if it's overdone which can also take some of the fun/challenge out of things.