The document discusses International Mobile Subscriber Identity (IMSI), which is a unique 15-digit number used to identify mobile subscribers. An IMSI contains the mobile country code, mobile network code, and mobile subscription identification number. The document then explains how dual/multi IMSI SIM cards allow subscribers to have multiple IMSIs from different networks on a single SIM card. This provides benefits like reduced roaming costs, increased coverage, and the ability to use one number for personal calls and another for business.