Comandante is a senior guerrilla rank in Libertad, above Mayor. Reaching it unlocks the Glorious Leader achievement, so it is the rank to aim for if you are hunting every trophy.
Keep earning XP after Mayor. Capturing FND bases, finishing Yaran Stories and story operations, and clearing side activities all count. Most players reach it naturally while finishing the map.
Mainly for the achievement. By this point you have unlocked the rank-gated weapons and gear, so the new titles are mostly for bragging rights.
Rank 1: Machetera
Libertad rank 1. Weapons and gear marked for Rank 1 can be bought from Juan and his Arms Dealers.
Rank 2: Guerrillera
Libertad rank 2. Weapons and gear marked for Rank 2 can be bought from Juan and his Arms Dealers.
Rank 3: Exploradora
Libertad rank 3. Weapons and gear marked for Rank 3 can be bought from Juan and his Arms Dealers.
Rank 4: Espía
Libertad rank 4. Weapons and gear marked for Rank 4 can be bought from Juan and his Arms Dealers.