Walls, floors, ceilings and roofs all permit us to define multiple layers to represent a system of parts that define the whole. One part of this is the Function of a layer. I'm referring to these as shown in this image of the layers in a wall type.
There are Five numbered/bracketed functions and a placeholder that doesn't really contribute to the wall (or other elements) thickness...yet. I can be hopeful can't I?
What do they mean and what do they do?
First...notice the numbers? Why are they numbered? The number represents a pecking order, the order that they occur during construction.
 Structure - last time I checked the structure of a building comes first so it is numero uno!
 Substrate - Yes plywood gets applied to the exterior side of a wood framed house next, similar for other substrates.
 Thermal/Air Layer - Insulation goes in now, on the face of CMU and in the stud cavities.
 Finish 1 - This is EXTERIOR, yes exterior so make your wall types with this and only use it for the exterior side of EXTERIOR wall types. Don't use it for your interior walls types.
 Finish 2 - This must be for INTERIOR finishes, YES, use it for interior partitions. Yes it is okay to use this function both on the "exterior" side and the "interior" side of INTERIOR partitions. It IS important to define which side is "exterior" on your partitions.
For example a shaft wall that has two layers of finish material on one side and a single layer of another on the inside. Which side should get the double layer? Exterior...interior? You decide and then stick to it, don't waffle, waver, equivocate or prevaricate!
[ ] Membrane - Poor membrane, doesn't rate a number but this is for Tyvek and others. In many cases it goes on both sides, vapor barrier on one side and air barrier on the other. I can never remember which so I'm sure somewhere a house is sweating when it should be breathing!?!?
 Structural Deck - It shares the number one with Structure because it is special. Floors have this one which is just teasing Revit Architecture (RAC) users, more on that in a bit. For Revit Structure (RST) users it is no tease. This allows you to have a pretty accurate representation of concrete over metal deck. You know, the profile is "real" and actually has "flutes" and the concrete fills the "flutes" and doesn't involve adding repeating details to sections and details.
They also play a role in how these layers in various walls relate to one another when they meet at an intersection. We don't really want a finish layer to punch through a structural layer. It ought to stop and respect the boundary of the structure, sorry...you cannot pass GO, do not collect $200. If you are careless with these you may end up with some interesting wall cleanup conditions.
As for Roofs I don't typically encourage people to build up multiple layer families for these. I've come to realize that most of these are better as individual elements so they can have different extents, imagine a Spanish tile roof for example. The tile nearly always finishes a bit forward of the roof rafter tails and substrate below. Separate elements means I can create a better representation of the roof.
No, these images are not the same roof. The first is from a project I was involved in a long time ago now and the second is just quick mock-up of two roofs with a separate offset for the upper "layer". I used join geometry to "blend" them together.
I have the same opinion for floors. I really like to encourage the RST team (everyone has a structural consultant using RST now right?!) to create the structural slab because the poor RAC user can't create the metal deck profile themselves, only the RST software can. RAC can show it however if it was built in RST. This way the RST firm "owns" the structural slab and I model the finishes with RAC. Now we can do material schedules for floor finishes easily and not worry about the extra layers in the floors. I do have to communicate with the RST firm about openings/depressions and the like but I should have been doing that all along anyway. Now I have a bit more motivation?
Having written this I have no objection to compound roofs and floors during schematic design. They actually shine then because you can create basic sections of the building and these preliminary roofs and floors define that "untouchable" space for structure and plenum which helps us plan our floor to floor relationships better.