It wasn't until the 7-01-10 update that this rank had any item unlocks, even after GP rare weapons update.
This is the last rank to have any GP standard unlocks. From Captain III, all unlocks will be either gear and/or GP "rares" (i.e weapon's in the MYST-G case).
As of the Combat Arms Reboot, this is the last rank which unlocks new rental items. Starting at Captain II, players will be able to rent all items in the rental category.