Bountiful Sunlight requires membership in the Princess's Guard covenant to use and gradually regenerates 10 HP/second for a minute, or 600 HP in total for the user and nearby allies; summons and enemies affected with Undead Rapport are included.
Bountiful Sunlight benefits from miracle synergy, allowing it to heal slightly increased amounts of HP each second for each level of synergy, maxing out at a total of 20 HP per second with level 4 synergy.
As a stronger version of Replenishment, Bountiful Sunlight trades much higher faith requirement, attunement slots, and covenant restriction in exchange for greatly increased HP regeneration. This miracle could be useful in PvE for counteracting the effects of poison and toxic, although it will not protect a cornered player much past NG+ due to the increased damage output all enemies gain. In PvP, Bountiful Sunlight could be used as a powerful way to passively regain HP over the course of a long duel, putting pressure on opponents to end fights quickly, and opening them up to mistakes.
Perhaps the biggest advantage of Bountiful Sunlight is its ability to heal allies. Bountiful Sunlight is an excellent miracle to cast right before or during a boss fight.