Some confusion seems to surround the notion of dwarf, semi-dwarf and full sized (standard) fruit trees.
Virtually all fruit trees sold these days have started out their life as a normal bud or twig growing on a tree. A nurseryman cut the bud or twig (now called a scion) from that tree and grafted it onto another tree called a “rootstock”. The top of the rootstock is removed so that the growing energy of the roots is transferred to the newly grafted the scion. From that single bud, a new tree is grown.
Keep in mind that it is the rootstock that determines the size of a tree; not the bud, variety or other factor.
Different rootstocks have different dwarfing capabilities, some will only allow the scion to grow into a very small tree that will stay small no matter how old it gets. Other rootstocks allow a bit more height in the tree, these are the semi-dwarf. Standard (full sized) trees are grafted onto rootstocks that have no dwarfing capacity at all.
When planting dwarf or semi-dwarf trees, it is essential that the graft union (the spot where the scion and root are joined) is planted at least 2″ above the surface of the soil. If the tree is planted too deeply and the graft union is below the soil surface, the scion above the union will sprout and grow roots. These new roots will over-take the dwarf rootstock and “cancel out” any dwarfing that may take place. That can be a real problem if you were planning on fitting a dwarf tree into a small area.
On the other hand, if you for some reason changed your mind about having a dwarf tree and decided you would like a full tree instead, you can just go ahead and plant the tree deep enough so the graft is below the surface.
Even with Standard trees however, it is a good idea to keep the graft union above the soil. Often the roots of the rootstock are hardier than the roots of the grafted portion. This can be a real advantage to the tree if it is planted in a less than ideal location.