The header conforms to the DIMACS standard which is as follows:
p xor <max-var-number> <#functions>
The letter 'p' must appear at the start of the header. The second header token, which in this case is 'xor', identifies the input format. The header contains two integer fields: <max-var-number> is the maximum variable number contained in the file; <#functions> is the number of Boolean functions contained in the file. The following is an example of a valid header for a file containing 97 functions composed from variables x1 to x56.
p xor 56 97