This particular product comes in a set of five and they are compatible for the R21, R51, microlite battery. They will fit the R21, R22, R51 as well as the microlite collars.
They would make good spares to add to in ground electric dog fence kits as a back-up for when the originals die. After all the dog training collar would be absolutely useless if the batteries never powered the collar receiver properly.
I like the fact that they come in multiples which will last a long time and would be especially useful to those with more than one dog.