Here is the basics, if you have a saved game for Rainbow Six: Vegas and Splinter Cell: Double Agent on your Xbox 360, you can unlock extra headgear.
The info from Ubisoft:
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Tom Clancys Ghost Recon Advanced Warfighter 2 Unlockable Headgear
GRAW2 features some exclusive bonuses that can be unlocked upon given conditions. Extra headgears can be unlocked simply by having a saved game of Rainbow Six Vegas and/or Splinter Cell Double Agent.
How it Works
GRAW 2 searches your Xbox 360s hard drive and if it finds a saved game of Rainbow Six Vegas and/or Splinter Cell Double Agent, it will unlock the multiplayer headgear. If neither game has a saved game on the hard drive, then there will be 4 headgear selections that are locked in multiplayer with a description about how they can be unlocked.
If the owner of GRAW2 has:
1) GRAW2 and NO saved games of R6: V and SC: DA, there will be 4 slots of MP headgear that are locked. There will be in-game text explaining how to unlock these 4 headgear.
2) GRAW2 and R6V. 2 sets of headgear mimicking R6: V will become unlockable in MP.
3) GRAW2 and SC: DA. 2 sets of headgear mimicking SC: DA will become unlockable in MP.
4) GRAW2, R6V, and SC: DA, they will have 4 extra headgear unlocked.
Christian Allen, MP Creative Director at GRAW2 developer Red Storm Entertainment, said:
A bunch of our fans not only play Ghost Recon, but other Tom Clancy titles as well. Having unlockable headgear in GRAW2 multiplayer is a way to reward them and to incorporate other Clancy franchises into our game.