SIM PIN and PUK
Updated
The PIN is a 4 to 8 digit code that can lock a SIM against use without it; the PUK is an 8-digit code that unblocks the SIM after too many wrong PIN entries, usually three. Both are handled by the card under ETSI TS 102 221.
In practice
IoT SIMs are normally delivered with the PIN disabled, because a device that reboots cannot type a code. If you enable it, the firmware must send the PIN at each start (AT+CPIN), or the SIM stays locked.