For example, if Roger Federer has a quarrel with ATP and decides he doesn't want to be an ATP member anymore, is he still able to register and play in Grand Slams? other ITF tournaments? And if he can, how his ranking (for seedings) will be calculated?
Anyone (excluding women) of legal pro age can play any tournament if they receive a wild card. So if he doesnt play ATP tournaments anymore then he would not have a ranking and would need a wild card to gain entry into any tournaments (even futures and including grand slams). He would also not be able to be seeded in any of the slams. Wimbledon used to be able to seed whoever they wanted based on grass court ability but I dont think they do that anymore after many players complained they were fixing the draw. Grand Slams follow ATP rankings although run by ITF and futures use ATP rankings even though they are run by ITF also.
so your saying if I dont win the next 8 events I participate in, and the ATP doesnt find out about me winning $2500 on Safin vs Federer 4 years ago I can recieve a wildcard into the US Open this year. Alright.