PIRUS, PRCUS, and AMDAR are tags in the raw data files of the PIREP. These tags should not be part of the report, but are occasionally erroneously added to some PIREPs.
PIRUS is a tag that describes a report that has urgent criteria (UUA) and PRCUS is one that describes routine criteria (UA). (AMDAR) is an Aircraft Meteorological Data Relay (AMDAR) PIREP. These are automatic reports made by commercial aircraft carrying more precise weather observation gear and they describe weather conditions in the upper levels of the atmosphere. The data from these reports are fed into NOAA and other worldwide weather service forecast models to improve weather forecasts.
See also article: Why is PIREP strange looking?