Removable User Identity Module
Removable User Identity Module (R-UIM, usually pronounced as "R-yuim") is a card developed for cdmaOne/CDMA2000 ("CDMA") handsets that extends the GSM SIM card to CDMA phones and networks. To work in CDMA networks, the R-UIM contains an early version of the CSIM application.
Source: Wikipedia — Removable User Identity Module (CC BY-SA 4.0)