In the code I was working with the tag was being used to define how the animal was being harvested, ie with hunting knife or butcherHarvest vs say a stone ax an alltoolsharvest.
I am no expert but i see Tag used to say if a recipe is learnable etc... thus i would define it as a way of setting parameters to use or get somthing.
Hope that helps maybe a more experienced coder can give us a better definition of the Tag .