Apple trees are grafted onto a rootsystem or rootstock of a type of apple that is selected because it partly controls the vigour and eventual size of the tree.
Some rootstocks also influence disease and pest resistance, while others are better at growing in more difficult or wetter soil conditions.
If an orchard of large standard trees is planned, then the vigorous M25 roostocks would be suitable.
For smaller areas in gardens or for trained fruit such as cordons and espaliers, then MM106 would be the rootstock to choose.
The Sussex apple varieties I grow are grafted onto the following rootstocks:
A good rootstock for bush apple trees or for trained fruit, MM106 grows well in a wide variety of soil conditions, though not in waterlogged soil.
Plant 5 metres apart.
A vigorous rootstock that produces a large tree that is seen in traditional orchards.
Plant at least 8 metres apart.