I genuinely can't and wouldn't know where to look , have put things into Google and a couple threads come up with fans saying what we are , they do and they don't .
Hard to find
I think it is just a handling charge they get, goes back years when the Man Utd's and Arsenal's of this world got really greedy and wanted all the cake, personally I think a team should get a % of how many fans they take to away games.