Format of Imp Unique ID

Hi! First post…so be gentle. I have been looking for something that states, officially, the format of the imp unique ID. From what I can tell, it seems to be 16 - 18 hex values long. I am looking to store this value in a table and was wondering if anyone has seen something officially stating the max number of values this ID will be.

The impee unique ID is always 64 bits long (16 hex digits).

Hugo…we can always count on you. Thanks!!