Some DirectX applications want to apply alpha transparency to the frames they output for the purposes of showing content underneath it. For example, you could write a DirectX application that acts as a dynamic overlay with animations and such, and capture it with game capture and enable alpha blending.
It's not an option that will be used very often, and if you're capturing an actual game, you will probably want to leave this option disabled.
Some DirectX applications want to apply alpha transparency to the frames they output for the purposes of showing content underneath it. For example, you could write a DirectX application that acts as a dynamic overlay with animations and such, and capture it with game capture and enable alpha blending.
It's not an option that will be used very often, and if you're capturing an actual game, you will probably want to leave this option disabled.