Hi all,

I have a Sunset Apple tree that we have inherited with our new house.
Its about five feet tall which, I understand from a few websites, is
about the size they supply plants to start with. I am also told that
pruned it should easily maintain a height of 6-8 feet (ideal for me in
its position) or 12-15 if left untouched.

Then it mentions that the tree is on a rootstock of MM-106

I understand what they mean by rootstock but exactly what is MM-106
and where did the numbering system come from?