Sargento is guerrilla rank 6 in Libertad. It unlocks another batch of weapons and gear for purchase from Juan and his Arms Dealers.
Your guerrilla rank rises as you earn XP across Yara. Each new rank opens a new batch of weapons and gear for purchase, and some items say right in their unlock text which rank you need. The game shows either the feminine or masculine form of each title.
Mid-game XP comes mostly from story operations and capturing the bigger FND bases. Spend your pesos at Arms Dealers as new stock opens up.
Rank 8: Capitán
Libertad rank 8. Weapons and gear marked for Rank 8 can be bought from Juan and his Arms Dealers.
Juice Injector
Boosts your speed and health and lets you perform Machete kills on any enemy, even in open combat.
Rank 4: Espía
Libertad rank 4. Weapons and gear marked for Rank 4 can be bought from Juan and his Arms Dealers.