The 3 Ranking Systems are:

1. EXP Ranking System (Gain EXP to rank up through ranks like Private, Specialist, etc. Once you rank up, you gain Spartan Points, so different from the Credit System, and I think you probally get 1 Spartan Point when you rank up.)

2. Specialization Ranking System (level up, your specialization to unlock "exclusive" armor and weapon skins, tactical packages and support upgrades, and emblems, then you can re-enlist to another specialization once you reach the level cap of your specialization, which is level 10, which is not your actual EXP rank, it's just your specilization level)

3. Trueskill ranking system. (not much is known about this, except your trueskill is only visible to yourself, and it's NOT the Arena system from Reach, and it's most likely NOT going to be 1-50)