Also, while FAR 121 and 135 operators HAVE to have the reported visibility met or exceed the minimums for the approach used, FAR 91 operators do not need the visibility to start the approach, just the "flight" visibility at the DA or MDA must be at or above mins.
In other words, reported RVR is 1800 feet, ILS mins are 2400 feet, the 91 operator could fly the approach, at DA, see the runway and land. The 121 or 135 operator could not proceed past the FAF with that reported visibility. I.E. there is no approach ban for US 91 operations. Yes, the FAA has violated Part 91 operators in a few cases.
GF