EDIT : the next answer is the reason why he will lose Doha, not Halle !
Duong, Why would Federer have to drop Halle? Don't quite understand...
as Slasher said, it's about the commitment : Fed will not play Tokyo or Beijing, which means that he will have only played 2 ATP500 tournaments in the calendar and will not be able to fulfill the comitment to playing 4 ATP500 tournaments in the calendar year.
Which means that he will get a zero for Tokyo/Beijing, just like last year.
PS : by the way, it's the same for Del Potro, which is why I took out his 150 points from Davis cup from my Race calculation, because he wil also get a zero for Beijing/Tokyo. Which is also why I said in this forum that Del Po was actually number 7 in the Race whereas he appeared as number 6 in the ATP's calculation.