I don't exactly know the answer to that, but there does seem to be a certain logic to it. The two big ones are Queens and Halle, and the tip-top guys seem to play one or the other, though there are a few others. The top guys are enticed (read: money,) and I believe they sign contracts. Halle, being in Germany, has always made sense for Federer, since it's pretty close to home, and he's a German-speaker. And I used to figure, in the height of the rivalry, that Nadal played Queens so they wouldn't have to meet before Wimbledon. However, Halle enticed Nadal away last year. I think Murray would catch flack if he didn't play Queens. Djokovic has played Queens in the past, though he's skipped a tune-up before Wimbledon the last 2-3 years. As to the rest, there is some level of invitation. You asked about Hewitt on another thread. He actually won Halle maybe 3 years ago, beating Federer in the final. And he has won Wimbledon. So, while he's generally past his sell-by date, Halle seems happy to have him back. He's still an audience draw, and better on grass than many.