The tag breast_grab should be used when a character's breasts are being grabbed in general, whether it's by a hand, a tail, a tentacle, or pretty much anything that can "grab" stuff, while hand_on_breast should be used when it is specifically a hand or hand-like appendage is doing the grabbing.
Examples:
post #947382 post #890722 post #981890
These posts get the tag breast_grab because their breasts are being grabbed, but the implication also gives the tag hand_on_breast, which isn't happening in these pictures, tentacles are doing the grabbing, not hands. I've noticed this every time I gave one of my tentacle pictures every time I put the breast_grab tag.
This is why I think hand_on_breast should be de-implicated from breast_grab, it isn't always a hand that's trying to cop a feel.
Updated by user 139414