You already answered yourself.
The main part of MSL is work with security flag to get access to hidden NVM units or to some additional service in AT commands.
Broadcom example:
AT+MSLADDSNUM, AT+MSLSECUR, AT+SECUNBLOCK
Qualcomm diagnostic example:
0xFA, 0xB4, 0XB5
But there many algorithms of MSL so no way to explain all of them
