The BOND system is a spiritual successor to the Confidant abilities of Persona 5. Rather than getting new abilities through ranking up with individual characters, the BOND system levels up the Phantom Thieves' ability as a whole, from increasing their physical capabilities, strengthening Personas, to various support mechanics.
Most skills can be increased repeatedly, up to a set level. Throughout the game, leveling up the BOND meter earns BOND points, which starts with a handful at a time. As the BOND level increases, the number of points gained rises with it.
There are 30 BOND skills in total, among which many are unlocked upon meeting set conditions, like the completion of a request, while others are tied to story advancement. Certain level caps can also be applied to prevent ranking up skills further, each capping out at different stages until certain story conditions are met or requests are completed.
BOND points are earned by progressing the BOND meter, which can be done in the following ways: defeating enemies in Jails, hitting story milestones and having social events with other Phantom Thieves, touching certain objects (mostly located near the party's camper, which can be used once per day for a small increase), cooking recipes for the first time, and finishing some requests.