Yup. Kind of cool that the arborists in the 15th century saw buds as "eyes"...it's like that little embryonic plant part was just looking at them and winking.
So the holes are filled with one part porous aggregate and other parts mycorrhizal material, such as soil and roots harvested from a healthy tree of the same species, and/or other bioactive material, so the mycelia of the introduced mycorrhizae and other microbes can graft onto the roots of the subject tree: