Advanced tab on Model Options dialog.
This is the maximum gap between neighbouring parallel surfaces that will be interpreted as being an adjacency between 2 spaces. It is used to determine which elements are adjacent to other spaces and which are adjacent to the exterior.
This setting applies to all model geometry but is particularly important when importing BIM geometry which often includes gaps between zones. You might enter a value higher than the default 0.5m if the adjacent spaces are separated by very large gaps due to thick partitions or ceiling/floor voids being excluded from any gbXML data imported. For example if the largest gap in the between spaces that are to be modelled as adjacent is 0.6m then enter 0.6m here.
The Adjacency tolerance can also be set from the Import BIM/gbXML Model dialog before finalising the import.