Access control cards/credentials contain chips that store data. The way that data is structured is called the "format" of the card. The number of ones and zeros, and how they’re put together, determines the format and ultimately the credential number.

For example, US phone numbers follow a well-known format: 9395981699 is recognized as 939-598-1699. Knowledge of the format allows proper reading. The format defines the bit length and fields of the credential number.

Every Format has a maximum number of bits – the count of the binary digits (zeroes and ones) that make up the credential. The available sizes are 26, 33, 37, 48 and 50 bit. The most common and industry standard card is a 26 bit card. This 26 bit format is recognized by all access hardware. The higher number of bits (33, 37, 48, 50) can increase card security.

Some of the higher bit formats are "proprietary", and usually carry a higher price tag. One exception is the HID 37 bit proprietary format, priced similarly to a 26 bit card.

HID - Understanding Card Formats