Top of the server means that there arent levels above you, your actual level is the top, next season you got one level up, but you continue o stay on the top.

Tanking means lose on purpose to stay 2 or 3 season in the same level, that way some managers will passe you, and you will have 2 levels of managers above you, that will sell there 3 stars, that you will see as 5 stars. By tanking your players will also be stronger, since they will not lose one star.