PID = patient ID (e.g. local identifier).
MPID = master patient ID (e.g. NHS number).
RID = research ID (hashed version of PID, consistent across database regeneration).
MRID = master research ID (hashed version of MPID, consistent across database regeneration).
TRID = transient research ID (integer having a 1:1 mapping to RID, that is convenient but can sometimes be regenerated).