The canopy of the tree.
I think we you know what branches are. Not stems, not the trunk, not leaves, not the fruit. Branches.
Also called the graft union, the bud union is the intersection of root stock and trunk. Branches form above this division and suckers form below it.
Rootstock is the part of the tree below the bud union. This is where you particular citrus cultivar was grafted onto. The rootstock does not dictate the type of citrus tree. Rootstock is chosen based on certain preferences. A dwarf or semi-dwarf tree is determined by the rootstook being used. Over the years, many types of rootstock have been tested out and some are more resistant than others. Some rootstocks are very common, most notably the ‘flying dragon’ variety which is a sweet orange tree.
The big mass of roots found under the soil. It’s a ball of roots. Rootball.