The mother rootstock is the one that all others ( normal ) come from.
This is to ensure that ever stock is true to the original.
The mother stocks are checked to ensure that they are geneticaly the same as the original parent stock so that no variations have happened in their breeding.
This is the same as is done with varieties every now and then to make sure that they are true to the origanal named tree.
So...if you take a Golden Delicious apple say...it will be geneticaly identical to the parent tree of 1890.
All nurseries will have their mother trees and stocks checked to ensure that the vars grow true.