Every card has a unique number to represent individual card holders. This is essentially the card's serial number or card ID number.

Your access control software will allow your system administrator to set building access privileges specific to each card's serial number. To get the most out of your system, you will want to take steps to prevent use of duplicate serial numbers.

With 26 bit cards (also known as H10301 format), there is a risk that card numbers are duplicated, making the dupe cards unuseable. This is because you, the end-user, are responsible for keeping track of all card numbers. This can be difficult when multiple locations or multiple people buy cards. However, some users like that all card numbers are sequential with 26 bit cards.
With 37 bit cards (also known as H10302 format), there is no risk of duplicating card numbers, because all the card numbers that are used within the 40,000,000 different possible combinations are documented. However, with 37 bit cards, although all of your card numbers within a single order will be sequential, there will be gaps in numbering across your entire card inventory, as cards are numbered as they "come off the press".
With 40 bit cards (also known as C10106 format), the start numbers are assigned, meaning there is no risk of duplicating card numbers because cards are numbered as they ''come off the press.'' All of the card numbers within a single order will be sequential, but there will be gaps in card numbers across your entire card inventory.

Start Numbers are only specified by the end user when ordering 26 bit cards.