I think "burl" properly refers to a portion of a tree (or big bush) that some agent, like insects, fungus or disease have invaded the wood and caused it to grown in a non uniform way, kinda like tree cancer I think is the best analogy. At least that's what I think it is.
Wood cut from these burl outgrowths can range from highly figured with lots of 'eyes' to plain or any combination inbetween. Since it is a non uniform growth it can have voids or all sorts of things.
I guess when all you have is one pen blank size piece of wood that is highly figured it's hard to know if it is actually burl. At least for me it is.
For example, Amboyna Burl. So they say, it is an outgrowth of Narra trees from the phillipines and SE Asia. You can compare the two and they are different, but come from the same tree.
This is just stuff I picked up googling around about wood. I try to make my products more interesting to potential customers by making this type of information available to them if they want it.
I could be completely wrong though. Ask my ex wife.
