so what does it do

Hello,

"Set FPS limit" function controls the frame rate of the target (game, video, and etc.)
1. Set FPS limit (60 FPS)
If you check only "Set FPS limt (60 FPS)", the maximum FPS of the target (green-colored numbers) will be 60.

2. Set FPS limit (60 FPS) only when capturing
If you check "Set FPS limit (60 FPS) only when capturing", the maximum recording FPS (red-colored numbers) will be 60.