The “detached” part of “detached structure” is easy to identify: If there is clear space between the structure and the dwelling, it’s detached. Structures without clear space separation are part of the dwelling.
Structures are things like garages, sheds, or gazebos. Some insurers consider fences or docks detached structures as well. A permanently installed object that’s meant to have people on or in it should be viewed as a structure.