When you build a key by defining a colour range, you can use one of four colour models: RGB, YUV, HLS, or RGBCMYL. Each model interprets the key-in clip differently and gives a slightly different result.
Once you choose a colour model, you set a range of colours to become partially transparent in the key-in clip to soften the transition between the front and back clips. This is called the softness range. All pixels in the front clip within the softness range become grey in the matte. The key should have the greatest possible softness value.
After setting the softness range, you set a range of colours to be keyed out in the key-in clip. This is called the tolerance range. All pixels in the front clip within the tolerance range become black in the matte. The tolerance range must not be too large or the edge of the composite will be too hard and the subject in the front clip will appear to be pasted into the back clip.