A big oak tree is made up of a main trunk and multiple branches. A "branch" always contains leaves, but may not always contain logs.
heightLimit: The actual height of the tree, including leaf blocks at the top (between 5 and 16)trunkHeight: The height of the trunkleafDistanceLimit: The distance from a log that leaves can generate, set to 4 for sapling growth, though appears to be set to 5 for natural growth for some reason