Under the Surface menu you have your Flat/bilinear/bicubic controls. Under Bilinear you have XYZ for each corner. The Offsets are just another fine tuning of axis controls for those points. The first offset XY points are for the entire surface... which cascades down to your main axis settings.
The second offset will control the position of the actual surface video in relation to the raster. You can select each coner with the blue pop up.
When you go to track something... the axis or offset keyframes will automatically be genrated and then you can use one or the other to fine tune your track.
So in the instance of a 4 corner track, you can track a box and then offset that track and or repeated on the other layers to make you only have to track once.