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@Maya, correct: the basement, ground and top floors are explicitly modelled, while (all but one) remaining mid floors are represented using a typical floor (usually mid-height) in combination with zone multipliers (here 10x):

With a text editor, check the .osm file for something like:

OS:ThermalZone,
  {cd521233-a2a6-482e-97ca-42d4a856eda3}, !- Handle
  Core_mid ZN,                            !- Name
  10,                                     !- Multiplier
  ,                                       !- Ceiling Height {m}
  ,                                       !- Volume {m3}
  ,                                       !- Floor Area {m2}
  ...

The use of zone multipliers is well explained here. Hope this helps.