What the 15 bits contain
The field starts with 5 data bits: 2 bits for the error correction level and 3 bits for the mask pattern number. Ten more bits of BCH error correction are added, making the code BCH(15,5).
The 15-bit result is then XORed with the fixed pattern 101010000010010. This keeps the field from ever being all light, which could confuse a reader looking for the edge of the code.
- Error correction level bits: L = 01, M = 00, Q = 11, H = 10
- Mask pattern: 3 bits, 000 to 111
- Error correction: 10 BCH bits
- Fixed XOR mask: 101010000010010
Where it is stored
One copy wraps around the top-left finder pattern, in the row and column next to its separator. The second copy is split between the area below the top-right finder and the area beside the bottom-left finder.
Two copies mean the code can still be decoded when one corner is damaged. Next to the bottom-left copy sits a single module that is always dark, called the dark module, which is not part of the data.
Why it is read first
A scanner cannot decode the data until it knows which mask to remove and how the codewords are split between data and error correction. The format information answers both questions, so it is read right after the finder patterns are located. Micro QR codes use a single 15-bit format field that also encodes the symbol version.