R

• Acknowledgement of receipt (message handling)

  General

• Received (acknowledgement of receipt)

  ICAO

• restricted area (followed by identification)

  ICAO

• right (runway identification)

  ICAO

• runway (used in RVR measurement)

  METAR

R has 5 meanings; the right one depends on context. A weather report, a NOTAM or an ATC clearance. Got a NOTAM full of contractions? Paste it into the decoder.

Source: FAA Order JO 7340.2 Contractions. Not for operational use.