co-author here, thanks a lot for your feedback! We will make sure to clarify on the apartment_id field in the next version. The apartment id is actually the unique id for an apartment in a single site (note: it happens an apartment spans multiple floors) - but it is not necessarily unique over the whole dataset.
We will also include a plan_id field that allows to identify which floors of a building are repeated (the apartment_ids differ already though).
We will also include a plan_id field that allows to identify which floors of a building are repeated (the apartment_ids differ already though).