From Halopedia, the Halo wiki
Commendations are awards found in Halo: Reach, Halo 4, and Halo 5: Guardians. Commendations are advanced through gameplay and provide bonuses such as credits, Experience Points and unlockables. Commendations are also intended to serve as an indication of how a given player tends to operate.
In Halo: Reach players advance commendations through campaign, multiplayer, and online Firefight gameplay. Players earn credits as they perform certain actions, many of which are Medals in and of themselves. Each commendation is represented by an icon plate. These commendations advance through five levels: Iron, Bronze, Silver, Gold, and Onyx before becoming Maxed Out. Upon advancing a commendation to a new level, players receive bonus credits. There are 16 campaign, 15 Firefight, and 14 multiplayer commendations.
Commendations return in Halo 4. Certain commendations in Halo 4 unlock armor permutations upon completion.
Halo 4 commendations are primarily advanced through playing War Games and Spartan Ops; only nine commendations can be completed in the campaign. Commendations are organized into five categories: Weapons, Enemies, Vehicles, Player, and Game Types. Weapons and Game Types commendations are earned only in War Games. Enemies commendations are only earned in Spartan Ops. Vehicles commendations can be advanced through Spartan Ops and War Games. Player commendations are all advanced in War Games save for the nine commendations completed by beating the campaign on Legendary.
There are seven tiers of commendations: Novice, Qualified, Proficient, Adept, Expert, Distinguished, and Master. Not all commendations utilize all seven tiers- the majority contain only five. If a commendation has only six tiers, it does not include Qualified. A five-tier commendation will also omit Distinguished.
Upon completing a tier, players are awarded bonus XP. The amount of XP varies from tier to tier and commendation to commendation, ranging from 250-10000XP.
Halo 5: Guardians
Halo 5 commendations are advanced by playing Warzone and Arena matchmaking; for the first time, there are no campaign commendations. There are five categories of commendations: Enemy, Weapon, Vehicle, Game Mode, and Multiplayer. Enemy commendations are only advanced in Warzone; all others can be advanced in both.
Every commendation, save for Mastery commendations, has five tiers. Completing a commendation (and in some cases, advancing tiers) awards REQ packs and XP. Compared to earlier games, commendations in Halo 5: Guardians feature much lower totals and as such are much more easily completed.