What is meant by plumbing connections at "different levels"?

Question

I'm reading the Indiana plumbing code, and section P3107.3 says:

P3107.3 Connection at Different Levels

Where the fixture drains connect at different levels, the vent shall connect as a vertical extension of the vertical drain. The vertical drain pipe connecting the two fixture drains shall be considered to be the vent for the lower fixture drain, and shall be sized in accordance with Table P3107.3. The upper fixture shall not be a water closet.
^{Italics theirs, Bold mine}

When they say "connect at different levels", does that mean on different floors of the building, or at different heights on the same floor?

In particular, I've got two new bathrooms, one stacked above the other. The sinks are one one wall (above each other), and the tub/showers are stacked on the other side (~7 feet away). Is this referring to combining the vents for the two sinks on the two different floors, or is this referring to combining the vents for the sink and tub on a single floor?

My intent at this point is to have individual vents for the sink and tub on each floor, combining in the ceiling, then going up through a common vent exit for each bathroom. (I'd love to combine all venting for both bathrooms into one roof penetration, but that's a different question.)

This is the general plan, looking from the side, of a single floor's bathroom. Does this "connect at different levels" apply at the vent junction labeled <-- Apply here?

                                                        | vent to outside
  ┌--------------- vent across ceiling (with slope) ----┤ <-- Apply here?
  |                                                     |
  |                                                     |
  |                                                     |
  ├--- sink drain                                       |
  |                         shower drain |              |
  |                                      └--------------┤
  └-------- sink drain (with slope)---------------------┴-┐ drain to basement

^{Assume traps exist at the drains. Work with me here...}

Answer 1

Your setup seems not to involve this bit. But same or different floors, it's the same.

Specifically at your "apply here" point what you show is a connection between dry vents, not fixture drains. Those generally are only limited to "6 inches above the flood rim of the highest fixture on the floor served" (as a minimum joining point height - also the minimum height where they can run "horizontal" [properly sloped])

If your sinks were near your showers so that the vent for the shower was also the drain for the sink (and thus, that section a wet vent) it would apply.

You show a dry vent for each of those fixtures. The sink drain eventually connects to the pipe below the shower drain, but the sink vent and shower vent are not impacted by that placement.

Given the limited DFUs of a lavatory (sink), you could very likely easily meet code requirements for wet venting the lower lavatory through the drain of the upper lavatory with a vertical wet vent. Stacked showers can also be done, but might (or might not, check the table) need a pipe size increase. As stated, a toilet/watercloset above eliminates the pipe below as a wet vent, so a dry vent (or wet vent through a different drain pipe) is required for anything below those.

My personal hot tip, which you probably don't want to follow if I recall your outside wall pipe placement issues, is to make all the small drains and vents 2" anyway, they are so much harder to clog - and buy you a lot of leeway in the wet vent line, too.