The octet length of the binary representation of the octet length of the payload.
Source(s):
NIST SP 800-38C

  When used as an FFC domain parameter, q is the (odd) prime number equal to the order of the multiplicative subgroup of GF(p)* generated by g. Note that q is a divisor of p – 1.
Source(s):
NIST SP 800-56A Rev. 2 [Superseded] under q

  When used as an ECC domain parameter, q is the field size. It is either an odd prime p or equal to 2m, for some prime integer m.
Source(s):
NIST SP 800-56A Rev. 2 [Superseded] under q

  Second prime factor of the RSA modulus n.
Source(s):
NIST SP 800-56B Rev. 2
NIST SP 800-56B Rev. 1 [Superseded]