Lawrence:
Here, is an example i created in hopes of it helping to make better sense of my words.

Firstly, about the 32x15, and 32x100 and such.
On the right side of the image I have dimensions shown.
Now, lets explain these alittle more.
The small iso tile that i have labelled 16x16 is the base standard tile. Since isometrics is rotated at 45 degrees to the viewer, so is its axis'. Therefore, X=16 and Y=16 (width x length). X and Y are no longer straight across and up/down because they are rotated with the tile to form this perspective. This is the flat part, what is parallel to the ground
The next tile I have is labelled 16x16x77
I already explained the first 2 numbers (16x16), so now ill talk about why i have 77.
77 refers to the height of the tile. This is the depth / z axis. Since isometrics is a 3d perspective, we now have 3 different measurments. The Z axis is perpendicular to the ground, adding height and depth.
But why is 77 the max for Z? this is because of the bounding boxs.
On the opposite side (after the equals) I have 2 square boxes. the first is labeled 32x100, and the second is 32x15.
The first rectangle is the bounding box for the whole 3d tile, 16x16x77. If you were to draw a box around that, it would measure to be 32x100. As I mentioned in an earlier reply, the max bounding box is 32x100. therefore, when you change it back to 3d, you have 23extra pixels on from the top and bottom diamond shapes. 100-23 = 77 is the max for Z, because then the whole tile fits within the bounding box.
The second small box, is the bounding box, 32x15 for the small iso tile.
Dont worry yourself too much about the bounding boxes. Those are just there for some people for an added visual, and for when i sometimes refer to the bounding box, or 32x15 for the tile(that being the same as 16x16iso).
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about the tree.
Originally, I had planned that only what could fit into the tile/bounding box would be in that tile, no overflow onto other tiles.
However, as i went to make you an example of what I wanted with this tree (tree from Super Mario RPG)
I found it to be quite difficult and hard to determine which leaves and branches go to each tile.
So, Ill make a rule correction.. Since I found out myself the difficulty and impracticality of my older idea.
For trees, or small bushes the leaves and branches are allowed to extend out over the 8 surrounding tiles, like i have shown. This will now only count as one tile, because the base of the tree is on 1 tile, and the rest of the leaves overhang onto the nearest tiles.
Is that better?