When a Japanese maple is a "cultivar" it means it is a clone of a unique tree that had some interesting/unusual characteristics - be it pretty coloring, interesting leaves, dwarf growth habit, whatever. All A. palmatum 'Beni Maiko' are clones of each other - and clones of the mother tree. They are all genetically identical. There are only four ways to clone plant material - by cutting, grafting, air-layering, or by tissue culture. For Japanese maples, the majority of trees are propagated via grafting - where you take a snippet (scion) from a mother tree and graft it onto the root stock of some other generic (ie non-cultivar) Japanese maple. The top of the tree is 'Beni Maiko', the bottom is generic roots. You can usually see a swelling or scar at the graft site on younger trees - which isn't a good feature in a bonsai tree.
"Standard" A. palmatum is simply seeds that randomly occur in nature. Imagine a nursery with 1000 large Japanese maples of many different cultivars. Now imagine an empty field next to the nursery - and the empty field is covered with thousands of Japanese maple seedlings from seeds that have blown over from next door. The young trees can be any combination of features from any of those cultivars, or perhaps look like nothing special at all. They are ALL generic A. palmatum. (without cultivar name) That doesn't mean that some aren't uniquely beautiful in their own way - perhaps even more beautiful than a cultivar. In Japan, they sow fields of Japanese maples and then hand select the ones that show characteristics suitable for bonsai. It is very unusual for any of them to be special cultivars. Some call these seedlings "generic green Japanese maple" just to make sure people understand they are maples from the field/nature/random genetics.
Dissectum is a type of leaf style for Japanese maples. It looks feathery versus the standard Japanese maple shape. It is not a good or bad thing - but there are many cultivars with dissectum leaves. Dissectum leaves because of their feathery nature are a little harder to work with for smaller bonsai trees. They also tend to be a little less sun and heat and dry wind tolerant.
Because most cultivars came from recessive gene expression, they tend to be weaker than generic Japanese maples from nature. That is why, for most grafted trees, the nurseries will use "green Japanese maple" as the rootstock (strong) in order to support the cultivar scion (weak). When I say "weak" some cultivars are weaker/stronger than others. Popular cultivars like 'Bloodgood' (probably the single most popular landscape cultivar) get at least part of their popularity from their strength and relative adaptability. That is why when someone refers to "intermediate in rooting", it could refer to strength of rooting, in general, and/or the likelihood of getting cuttings to root. Some JM cultivars can be propagated by cutting relatively easily... others that are weaker are very difficult if not impossible.
