I'm think this is where there maybe some confusion? NIMS uses Division and Groups as a way to further divide a Section or Branch. Divisions are used when you assign this new "section" by geographical means, Group is used when you do it by function. The guys assigned to to vent the roof could be either the Roof Division or the Ventilation Group.I think that there is some confusion about the application of NIMS/ICS terminology. The term Division is a functional position in the ICS. .
This is why NIMS may not be suitable for every day ICS. Why try and apply some confusing division assignment for something so important. Use what your FD trains and knows. Do you need a "Division" level assignment for a crew or two working on the second floor? How many crews are reporting to the "Interior Ops" officer? Applying multiple levels to meet NIMS is misaligned. I have yet to see where anyone says we must use these terms for every geographical area of any operation? Anyone? NIMS is an ICS tool, not an ICS system and does not dictate tactics or operations. People are taking this far to far, to the point that it will create confusion at the worst possible time. Sorry to rant.I understand that a Division is a level in the command structure, but lets just say you have a incident that requires the IC to break up the interior operations into Divisions on each level following ICS 100. So there is the question, if you are in a large single family home (like some that we have in our still district up to 20,000 sq ft.) and these homes tend to have weird floor levels. You may go up a flight of stairs to a landing and go up different flights from that landing to other levels the split. Or like William stated in the question in a three step ranch when you go up the three steps to get to the bedroom locations are you on a second level which would make you Division 2 per the ICS 100 glossary.
My opinion is we can not prepare for every floor plan or house layout. I feel that it will take a strong command presence by who ever is going to be the first (of what may be many) Division leaders, to make it VERY CLEAR that the structure that the incident is happening in has a different floor elevations than the "normal house". That person will set the tone of how the structure will be divided for the rest of the Divisions in that building. When that is made clear on the fireground hopefully that will get everyone on the same page.