Daryl wrote on Monday that the concept of View Discipline isn't documented well. Software documentation tends to be a bit dry, too often a very literal, "Un-checking this turns the feature off". Well that is obvious but "why" would I want to turn it on or off? Unless it is truly self evident, why a feature exists is a bit more involved but much more satisfying help.
The concept of View Discipline appeared with Revit Structure. It was meant to make it simple to alter the appearance of our model to be more in sync with what a structural engineer wants to see. When MEP (Systems as it was called then) appeared a year later they extended the concept to Mechanical and Electrical (plumbing lumped in with Mechanical till release 2013).
Daryl used plan views as his example of how the help documentation isn't accurate. Plan views are not the best view type to see the truth in the help claims. Ducts for example, he wrote, don't show up when the architectural discipline is chosen. They CAN show up, they probably won't be visible because they are above the cut plane. When Mechanical is assigned Revit changes the nature of what plan and reflected plan mean. Ducts at 10'-0" AFF will appear in a plan view with the view discipline Mechanical assigned. They won't with Architecture. They are there, just above the view.
This subtle change occurs because the notion of a plan and reflected plan view means slightly different things to architects and engineers. A reflected plan, architecturally, means that elements that are lower mask elements that are higher, such as a ceiling and light fixtures mask elements above that ceiling. A HVAC plan is different because most engineers want to see the highest duct masking lower ducts. It isn't a reflected plan, it is a plan whose cut plane is, in a way, altered to be above the highest elements and "look down".
Technically the view range settings can be identical but switching to mechanical discipline will show ducts that weren't there a moment ago when architectural was assigned. They were there but the discipline change altered how Revit calculates the display of the elements. Engineers don't generally do reflected plans so Revit made it easier to generate regular floor plans that show their elements in a way they expect to see them.
The combination of View Discipline, Object Styles, Visibility/Graphics Overrides and View Range are all responsible for what we see in a given view (technically there are more). Changing one of them is not going to ensure that we see what we really want. We've got to make sure they all have complimentary settings. There is no real difference between Mechanical and Electrical disciplines graphically, based solely on the View Discipline parameter changing. The views dedicated to each of those disciplines also use V/G and possibly view range settings to make them look correct. If you examine the stock templates you'll find that these views have each others element categories turned off (with V/G) to achieve the look we are after. Apart from the automatic graphically biased changes they make, they really just help us segregate views in the project browser.
In my own way:
Architectural - All model elements treated "equally", according to Object Styles, but with architectural bias with respect to view range and cutting elements.
Structural - Similar to architecture but hides non-load bearing walls and some hidden line behavior with respect to concrete floors/slabs and foundations.
Mechanical - All other disciplines are half-tone and transparent (help says "on top"), duct and pipe and related element categories take priority and behave according to Object Styles. Mechanical elements like duct and pipe will appear in plan regardless of their true elevation with respect to view range, as long as they are within the Primary Range.
Electrical - Same as Mechanical.
Plumbing - (new to 2013) Same as mechanical.
Coordination - Same as Architectural but provides for segregation from it for easier management of views.
Then there is the Sub-Discipline parameter, a Project Parameter that first showed up in Revit MEP templates. I see architecture firms, that have been exposed to RME, using the same parameter now in their architecture templates because of the added (for consistency as well) control over the Project Browser sorting. That's the essence of the parameter's existence, to manage the Project Browser according to the kinds of views each discipline typically creates. Such as Power and Lighting plans for electrical and HVAC Duct vs HVAC Piping.
Keep in mind that views in stock project templates are also configured to only show certain element categories according to the discipline bias implied by their name and discipline parameter setting. The combination of the View Discipline parameter and Visibility/Graphic Overrides is responsible (among other possible view properties) for the final result of what you see in a view.