Hi there, for my job, I try to modell the floor plan of the Rostock University of Music and Drama which is a building basing on a old monastery. This makes the modelling a tip more complex than for usual and this is my first indoor mapping project, even when I’m in private a long time 3d mapping entheusiast 
So I started with level 0, which looks fine to me. Unfortunatly, some areas of the older building parts have a intermadiate level. This is normal used levels whith rooms and floors connected by steps and elevators, but with dedicated label (german: Zwischengeschoss ZG) and cover only some parts of the whole building. This brings up the question, how you would model this with Simple Indoor Tagging and keep a maximum compatibility to make the data available to all the diffrerent tools to visualize / routing / …
What I read is to use also level:ref=* tag to introduce the custom level label “SZ” beside the level=* numbering. But is using level=2 for the ‘real’ level 1 not pretty confusing? Or is it encouraged to use level=0.5 for this added levels and comes with good compatibility?
I would love to hear some advices from the indoor mapping seniors 