O -> top tree layer
________/|\_________
/ | \
O O O -> sub-tree layer
/ / \ \ / / \ \ / / \ \
O O O O O O O O O O O O -> base layer trees
Each tree layer has its own arity BaseTreeArity, SubTreeArity, TopTreeArity.
The tree above for example is a tree constructued from 3 subtrees with a BaseTreeArity of 4.
Its also important to understand that trees are stored in reality as flattend trees basically a array of elements.
You can learn more about these trees here https://github.com/filecoin-project/merkletree.