The difference between holding_* and *_grab tags is nonsensical.
Right now there's a distinction made that "grabbing" means "another character is touching that body part", and "holding" means "the character is touching their own body part".
Unsurprisingly, the results are that these tags are constantly misapplied (For example, there are 3.7k posts under butt_grab solo and the same number under breast_grab solo -disembodied_hand), the distinction is entirely arbitrary, an uploader or tagger wouldn't know it without consulting the wiki page. Most logically take "grab" to follow the dictionary definition of "to grasp or seize suddenly and roughly", while holding is "to carry or support". Ask most people, and they'll say they are two different actions.
Is this really a "butt grab" when the character is holding up the other character by the butt? Or a "balls grab" when she's clearly supporting the weight of the balls with her hand and not "grabbing" them?
They should both be holding_*.
Is this better described as the character "lifting or supporting" their butt when they only have a single hand squeezing the top of a butt cheek? Would you say the character in the first image is just "holding a breast" or is she "grabbing her breast"?
They should both be *_grab.
We have multiple hand_on_own_* and hand_on_another's_* tags that could be used to make a clear, objective distinction of whose hand is doing the touching. We don't need to rely on an arbitrary distinction between *_grab and holding_*. We should use those, change posts using grab/holding to whichever action is more appropriate, and fix this issue.
Thoughts?
Updated