From Halopedia, the Halo wiki
Career Milestones were Halo Waypoint's overall measure of participation and performance across all Halo games released from September 2007 through 2014. Career Milestones were removed from the web version of Waypoint on October 13, 2014, and replaced by a simple Gamerscore total (which itself was discontined in 2021). They were still visible on the Xbox 360 version until it was shut down on January 13, 2022.
Milestones were reached by attaining a certain level of Halo gamerscore. When designated Halo Gamerscore levels were reached, new Milestones were earned and awarded in the Career section. Players began at Milestone 1. As new Halo achievements were earned, players leveled up their career milestone. Each additional Milestone beyond level 16 required 100 Gamerscore to unlock; earlier Milestones required fewer.
There were originally 40 milestones available at launch. It took 8900 Halo Gamerscore to reach the final maximum milestone 100. All those on a player's friend list were 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.