In the current implementation of platform security, UID3 is used as the SID if SECUREID is not specified in the MMP file. However, the SID is used to determine which private directory a process can access as well as to identify the caller applications, whereas the UID is used to uniquely identify an application. It is theoretically possible that these two identifiers could have different values, but to avoid confusion Symbian recommends that a SECUREID not be specified in the application's MMP file; UID3 should always be specified instead.