Traditionally before there were rootstocks the apple trees would grow to an extremely large size. Apple rootstocks were developed so that trees could be grown for all different situations Apple rootstocks are important as they determine the size of the tree. The young wood of the cropping apple is grafted onto the rootstock, usually just above the ground level, in the tree nursery.
Crab apples are sometimes grown on malus seedlings whereby very large trees are required such as in parks, but generally, they are not suitable for gardens, therefore ensure they are grown on a rootstock as for other apple trees.
In some situations, it may be desirable to keep the roots from spreading and Rootex bags are very effective means of control. See Root control with Rootex
When buying an apple tree it will always tell you which rootstock it is grown on. Below are the most common ones to consider and where they might be used: