SIRI is for realtim, so you can't really compare it to GTFS, but more to GTFS-RT
SIRI is implemented as a set of service, and each request can have some parameters, so you define the scope using the parameters to your request (available parameters are of course different depending on the request: stop, line, area, time interval, etc.).
Also note that most SIRI (and NeTEx) objects have a location structure information. In
SIRI the information about stop point location can be received thanks to
the StopPointsDiscovery however it's usually more likely hat you
receive them previously via NeTEx StopPlaces (remember that SIRI is for
realtime and NeTEx for scheduled information: SIRI Discovery services
are available for convenience when you don't have a NeTEx dataset, but
it only provides a very limited set of information).
Realtime location information (like vehicle location) is of course part of SIRI messages.