It's a stationary/round trip/circular slackpack really. Sometimes hikers use a service to go both forwards and backwards along the trail, returning to the same point where their pack is. I have seen this offered as slackpacking. The intent is to drop all those items that you expect to not need on a day hike that you are ordinarily carrying for overnight. Semantics aside, the gist of it is the dropping of weight which is sometimes criticized negatively.

For a NOBO or even a flip-flopper whose last day is on Katahdin, shouldn't be a big deal physically whether you use a day pack or your regular pack. If you are a SOBO with little hiking experience or understanding of a profile map maybe consider taking a day pack. Not saying you will get your ass handed to you if you don't, but heading home after your first day, well, that would be unfortunate given your hopes and dreams.

There's no official pack weight on the AT.