See Modern Arboriculture page 147. Callus - cells are all the same size and shape. Callus - has very little lignin. Callus - is meristematic. What this means is that it can divide and differentiate to form any and all parts of a tree.
Callus starts after a wound.
If you look you will see they come out from the callus ring. Now, As the tissues become lignified, as they begin to have different shapes, as they no longer are meristematic, Now, That which was once called callus, is no longer callus, and is, and should be, called, woundwood. Now, Dormant buds can form the second year on a sprout that has come from callus.