MSL code

MSL

Master Subsidiary Lock code (MSL) is a 6-digit code unique to your CDMA phone that's necessary to access some of your phone's deeper functions, such as wiping a phone number or manually programming in a new one.
GSM phones do not have MSL codes.

Z3X needs to bypass MSL to be able to modify any setting, such as have write access to the EFS partition and change IMEI/unlock the network, etc.