What does the T in RVR-T stand for?
From page 5-1-7, contractions in the AIM:


  RVRT . . . . . . . . . RVR Touchdown


The full list is:


  RVR . . . . . . . . . . Runway Visual Range
  RVRM . . . . . . . .  RVR Midpoint
  RVRR . . . . . . . . . RVR Rollout
  RVRT . . . . . . . . . RVR Touchdown


On some runways there are multiple RVR sensors and different points along the runway, and these are the names given to identify the different locations.