How many times a second FixedUpdate runs

If the frame took longer than a FixedUpdate step, we need to run multiple steps for that frame, to catch up. How many are allowed? Too few, and the simluation will run slower than the game. If you allow an unlimited amount then the frame time could snowball to infinity and never catch up.

If you're seeing objects go through other objects or you have a low tickrate, you might want to increase the number of physics substeps. This breaks physics steps down into this many substeps. The default is 1 and works pretty good. Be aware that the number of physics ticks per second is going to be tickrate * substeps. So if you're ticking at 90 and you have SubSteps set to 1000 then you're going to do 90,000 steps per second. So be careful here.

If false, then instead of operating physics, and UpdateFixed in a fixed update frequency they will be called the same as Update - every frame, with a variable time delta.