MM.106 clone 9a is a semi-dwarf rootstock. It is a productive rootstock and can be freestanding but support is recommended when trees are young.
The MM106 is commonly used, and still the standard in, for example the professional apple cider industry or dryer climates like the Mediterranean or Northern Africa.
Fairplant is producing the MM.106 clone 9a which is the best MM.106 clone on the market due to his higher tolerance for poorly drained conditions than the standard MM.106 clone.
This rootstock may be a good choice for home gardens as well.