What is the deal with flights that stop, then re-board?

I just saw an American Airlines flight from PHX to CLT that stops in DFW. The layover is 40 minutes. Then, it’s back on the same plane, same flight number, same crew, everything. It’s already on the tarmac, why are passengers disembarking and re-boarding? Is this common?<br><br>Edit: I think I figured it out. It’s the same plane, different flight legs. It originates somewhere else, stops in PHX, then heads to DFW. Then it is a CLT flight, after a 40 minute layover. Some flights have a single flight number and you’re not allowed off the plane...these are called through flights (something I just learned), but this isn’t that.<br><br>Edit2: when this flight is fully booked, how do they fit 240 passengers, plus the 240 passengers that then board in DFW?<br><br>Edit3: I just looked at a specific flight. Departing PHX with 175 passengers, that flight was 40 minutes in DFW. Then 179 passengers left DFW later that day. 175 passengers could not get off that plane in 40 minutes, and even if they did, there’s no room for the other 179 passengers in DFW. Suddenly, the plane is over capacity.<br><br>Edit4: I was reading the comments on here and I finally think i figured it out. The first leg of the flight is the main route, the second leg of the flight is likely a feeder to a larger airport. For instance, the PHX leg is probably full the second leg is probably 1/4 full or less.