The concept of Type parameters vs. Instance parameters seems to put many users' minds into a spin. Instead of trying to describe what the difference is, I'm going to provide a scenario where Type parameters should be used instead of Instance parameters. Perhaps by providing a concrete example, one will be more readily able to discern when to use one over the other.
I came across a Revit project file recently. Some things weren't jiving for me in trying to diagnose an issue, so I put together a quick schedule, and found this:
If you have a keen eye, you're asking yourself, how can multiple "Fixture Types" be associated with the same Family and Type (i.e, the Corp_Recessed : 277V Recess)?
Upon further inspection of this particular Family, I found the following:
The issue is simply that the method used in this project to 'type' the fixture will easily lead to inconsistencies. The Instance parameters, indicated by (default), introduce a lot of uncertainty into the model.
Take for example the following instances in the model:
I.e., what is to keep someone from adjusting the length of an "A1" fixture to be 8' long instead of 4'? What is to keep someone from adjusting the geometry of an "A4" fixture to appear like a 2x4, or even a 4x4 (instead of the apparently intended 2x2), or adjust the height too? Or... what is to keep one from mis-'Fixture Typing' a "A1" as an "A9"?
Even the Load in this family is an instance parameter... I don't think anyone would want to rely on someone to enter the proper load at each instance of all of these fixtures. For this one type containing 514 total instances, that is over 2500 opportunities for error (514 x 5 params each: length, width, height, load, fixture type). Using Type parameters for these properties avoids all such problems and will lead to a much more consistent model.
Take for example (referring to the schedule above) fixture type "E1" - is this fixture supposed to be Wall mounted or Ceiling mounted? 120v or 277v? If it makes a difference in the model number that needs to be ordered, it could be a change order waiting to happen.
To summarize: each fixture type (corresponding to a manufacturer's model number) of a lighting fixture should be uniquely identified - this is nothing new. However, the execution of how you do this can make your documentation much more consistent if applying the proper techniques in your modeling application. In Revit - this means make sure you have a separate Fixture Type for each Type of fixture. If you use the 'Type Mark' parameter built-into the software, Revit will even warn you if you in-advertently duplicate a value!
Martin Schmid, PE
MEP Customer Success Engineer