A scanner is set for receive mode only and can not transmit on any system in any way, shape, or fashion. It does not have any transmit capabilities built into it.
A subscriber radio is a two-way radio that is authorized by a system administrator to transmit on the system by having the individual radio registered to the system controller. Normally this is done with a valid registered radio ID number, and in most cases now, an electronic radio serial ID number to match the assigned radio ID number authorized by the system administrator of the system it's going to transmit on. A subscriber radio is a radio that is built to a specification of what the system entails (frequency band, features, suite of standards, etc.) and is far more superior in most cases to a scanner. However, as of the past few years, certain scanners have become almost as good or just as good as a two-way radio. These include the Uniden SDS series scanners and the Unication G1-G5 series pagers.