A camping reservation in the park gets you a parking spot at the campsite you booked. Sounds simple but there are only three campgrounds with parking lots that access the trailheads for Katahdin and only one campground, Katahdin Stream that accesses the AT. There are limited parking spots at each campground and they are managed fairly closely. If you are camping in the park at one of the many other campgrounds (some of the northern campsites are 2 hours from KSC), then you have to get up real early and drive over to the trailhead parking lots and grab a spot before the Day Hikers drive in the park. People who just want to day hike have to buy a day use permit for a specific lot on a specific day. They are guaranteed a spot to park. Its highly likely on a weekend that the Day Use permits will sell out and the three katahdin trailhead parking lots will be full quite early, in that case a camper inside that park can and will be turned away when the lot is full. The Togue Pond gate is usually opened about 1/2 hour before sunrise so those in remote sites may have to get up around 4 AM to ensure they beat the day use folks. Go during a non holiday weekday and you have much better odds.