From Halopedia, the Halo wiki
Career Milestones were Halo Waypoint's overall measure of participation and performance across all Halo games released since September 2007. While they are still visible on the Xbox 360 version, Career Milestones were removed from the web version of Waypoint on October 13, 2014 and replaced by a simple Gamerscore total.
Milestones are reached by attaining a certain level of Halo gamerscore. When designated Halo Gamerscore levels are reached, new Milestones are earned and awarded in the Career section. Players begin at Milestone 1. As new Halo achievements are earned players level up their career milestone. Each additional Milestone beyond level 16 requires 100 Gamerscore to unlock; earlier Milestones require fewer.
There were originally 40 milestones available at launch. It currently takes 8900 Halo Gamerscore to reach the maximum milestone 100. All those on a player's friend list are able to view the player's milestones.
Halo Spartan EVA Helmet - Make it into Career Milestone 50.
Halo: Reach Armor Unlocks
Military Police: Base - Career Level 15. Earn The Soldier We Needed You To Be achievement in Halo: Reach.