Fighting games like SF2 usually work on 2D hit boxes. [1] You have boxes which are defined as things that do hitting, boxes which are defined as areas that can get hit, and boxes that define clipping. If a do-hitting box overlaps a get-hit box, a hit occurs. It's a simple, elegant system that's ideal for a game with a fixed 2D playworld and fixed 2D attacks. It doesn't work for a system with free form 3D sword combat. If you block up and to the right, and a sword comes in from up and to the right, you should block it, but if it comes from up and to the left you should get hit.

https://www.youtube.com/watch?v=SB2CvPt87X8