The header conforms to the DIMACS standard which is as follows:
p cnf <#vars> <#clauses>
The letter 'p' must appear at the start of the header. The second header token, which in this case is 'cnf', identifies the input format. The header contains two integer fields: <#vars> is the number of distinct variables contained in the file; <#clauses> is the number of clauses contained in the file. The following is an example of a valid header for a file containing 97 clauses composed from 56 distinct variables.
p cnf 56 97