Air rights, when associated with a 'strata lot', can be an exclusive right conveyed on that lot to utilise an area outside of the strata lot usually acknowledged by a special by-law giving 'exclusive use' alternatively, the roof may form part of the strata lot or be a completely separate strata lot. They could also form part of a stratum lot thus allowing further development and strata subdivision)
The developer, when registering the strata plan, may include the roof as part of a lot or create a separate lot for the area. Otherwise, if it is generally accessible, may be part of the common area. In this case, someone may need to apply for 'exclusive use' & possibly pay for the privelege.
If the lot has air rights, there may be conditions as to the extent eg how high above a datum point the right extends (this may have been determined by a development application limiting the height of the building and any structures).
Air rights do have a value attached to them mainly dependent upon the usage to which the space may be used – eg installation of telecommunications panels and associated equipment (shelter, cabling etc) or signage. Neither of these uses have much value unless there is a demand for such items eg lowset building, quiet street, no exposure, poorly located etc.