The Components and Order of Layers.
As many battlemaps may be, impassable, non-combat details are above the grid as they cannot be interacted with - there is little reason to illustrate the distance across a solid, impassable wall, because you will never have need to measure across it. No one can move over it, and the chances of someone shooting over it are unlikely. Inversely, things that affect movement, distance, or behaviour are below the grid - for calculating range, movement paths, and token placement on the map. The Styx Oarsman shows this in its own way, along with many other WotC Maps, by following this, apparent, order.
Notes - (M) and (R) on the map, the arrow pointing to the forecastle stairs.
Doors - These are placed on top of everything else, to ensure visibility. Walls are choked beneath them to emphasise them further.
Walls - These stand over most other layers, to ensure both props and the floors are limited by the most 'immovable' of objects - the walls themselves.
Soft Barriers - These are such things as the curtains, high in the chain for much the same reason as the walls.
Combat-Affecting props - Whether they be heavy and tables that must be moved around, support beams to use as light cover, or benches and bars to hide behind.
The Combat Grid
Floor-affecting props - Trap doors, special terrain such as gravel, uneven surfaces and grates.
The Outside Land.
Now, alot of this so far probably seems self explanatory - ultimately, it is - but this wouldn't be much of a study if we didn't look at the details from the top down and find out how they work together in full, even the obvious ones.
Now, looking above, we can see that this arrangement is also a list of priorities. Players are less likely to be miffled by a lack of an identified floor, than they are about enemies popping through a doorway that was hard to see or never drawn. Likewise, the walls and potential paths within the building offer more opportunities than whatever is outside and likely exists merely to 'flesh out' the visual presentation of the map moreso. Likewise, notes, numbers, and markers stand above everything else, to ensure a DM or Player can find them quickly and more importantly - read them.