The rear part of boxer shorts can be sewn in four different ways. For example, Berg & Berg boxers feature a back panel made of three pieces of fabric, there’re side seams with side vents:
Sunspel, Derek Rose and Turnbull & Asser use a different technique. Their boxers don’t have side seams, but they’re equipped with an inserted back panel which looks like this:
Emma Willis offers boxers which feature a back panel made of two pieces of fabric, so there’s a center seam. Some people think it’s uncomfortable, and I tend to agree with them… but it’s interesting that Emma Willis boxers are very comfortable to my taste. Probably this is an exception – and of course, it may not work for you.
The fourth technique is used by Budd and maybe some other brands. Budd offers boxers with side seams, but the rear part of these boxers is made of one piece of fabric. They claim that this is the most comfortable option, but I would say that the first two options are very good too, and the third one (with a center seam) is controversial.