shay2501,
answer is dependent. it's an id you agree to use to identify yourself to your tp(s) but you also need the qualifier.
for example:
01 means you're using your duns number.
zz means you're using something mutually defined.
12 means telephone number.
14 means duns number with a suffix (if i remember correctly).
so you can see that they are unique numbers and it's in your and your tp(s) best interest to have them unique, so data does not get misrouted.
it's basically an identifier that you and your tp agree upon and that does not cause an issue with your VAN.
regards,
longhair