RVR is measured by visibility sensors. This document shows how they can be located: http://www.faa.gov/documentLibrary/media/Order/6560.10C.pdf
A controlling RVR means the reported value of an RVR reporting location (touchdown, mid-point and stop-end) used to determine whether operating minima are, or are not, met.
http://www.casa.gov.au/wcmswr/_assets/main/download/caaps/ops/lvo-1.pdf
Advisory RVR just for additional information gives visibility in other runway locations, decisions to clear for approach and land would be made regardless of it.