An x % fade will dictate the 2nd half to be x percent slower than the average equal pace for that half i.e. half the time of the full race.e.g. The two halves of a 3h20m marathon should take 1h40m each i.e. 100 min. A 2% fade will mean that the 2nd half should be 2% of 100 min i.e. 2 min slower.So the splits for the halves of a 3h20m marathon would then be 1h38m and 1h42m, give or take a second or two due to rounding and other complexities.A different topic is the fade structure or how the additional time in the second half should be allocated to the various km's in the second half.The pace charts may just have the middle ground on this that may on average work for most people.

A zero fade will have equal effort for the full distance. This mean the time for the two halves will be the same if it is flat or if the 2 halves have similar profiles. But some of us cannot maintain an equal pace over the full distance. We may not completely distance fit, we may start too fast , it does not really matter what the reason is. A fade allow for a slower 2nd half. It helps you to a target if you know you are going to fade i.e. run slower during the second half.