Appropriate authority policy references
Intruder is correct in that these are some additional appropriate recent policy references to consider, ...but note that Op-Specs still are the controlling document for a particular carrier.
Note that "Policy" (of an authority) as for Orders or Notices is not necessarily a "rule" and they may not necessarily be applied to a particular carrier or situation (as would be an actual FAR such as 91.175 or 121.651), ...and even Advisory Circulars can sometimes have alternate means of compliance (I can cite many examples), or delayed or revised updating of compliance of [Automated] Op-Specs, or a different means of compliance, or even alternate compliance (especially relief from "Notices" and "Orders", which typically are targeted to and pertain to direction of "FAA employees" as to being instructions (e.g., to POIs/CMO/APMs... inspectors), and not necessarily applying to the general public (except as indirect information). Bottom line is that the operator's actual approved signed Op-Specs rule here, and variances from both Orders, Notices, and even ACs can exist (and even differences from rules, if there are "exemptions" or "deviations" legally approved).
Regarding "relevant RVR" the language was originally addressed in AC120-29A and AC120-28D (with historical precedent as far back to AC120-28B (Dec 1, 1977) ...to cover situations such as two versus three transmissometer runway installations (8000' ft runway length breakpoint), temporarily inoperative RVR components, and later recognizing short landing distances of some aircraft types like the DHC-8 (with HUD) that could often accomplish an entire landing (or takeoff) within the relevant portion of just one installed transmissometer. The criteria here for short field length aircraft was keyed to conservatively slowing to a "safe taxi speed" considering mu and normal breaking, as well as non-normal events, ...and for takeoff, the distance for the entire takeoff, including any potential entire RTO at the relevant mu, if a stop was required.