Makers of push-to-connect fittings sell disconnect clips and disconnect tongs. The clips are $2, the tongs $8: what does the tong do that the clip can't?

I don't know that there is any difference except the price.

when I need to disconnect a fitting I just use a plastic j-hook. I keep an assortment with me.

This is a "clip"



This is a "tong"



With the clip you need to place it on the copper pipe at the joint then apply some force to back the coupling out of the pipe. Usually that means putting two finders over the clip then kind of a combination of turning and pulling apart will do. However, if the joint is say 1/4" from the wall, there may be no space to slide the clip into the gap, or if there is, no space to slip your fingers behind that to do the twisting then pulling. In that case a tong may be useful.

But as Packy said, neither of those are really needed as long as you have something to push that collar into the fitting to release it. I have the 1/2" and 3/4" clips but if it's real close I would use an adjustable wrench or a retaining ring plier to do it whatever is laying around.

IF there is room enough between the fitting and the wall for the clip, then there are other tools that will work. IF not, then the tong will slip in and give you mechanical leverage to depress the ring.