Type A Curves of Supersinglular In the Type A curve used in Pairing based Cryptography, what is meant by sign0 &amp; sign1 in the expression below?

a_param struct fields:

exp2, exp1, sign1, sign0, r:
  r = 2^exp2 + sign1 * 2^exp1 + sign0 * 1 (Solinas prime)
q, h:
  r * h = q + 1
  q is a prime, h is a multiple of 12 (thus q = -1 mod 12)


Reference: http://crypto.stanford.edu/pbc/manual/ch08s03.html
The values $\mathtt{sign0}$ and $\mathtt{sign1}$ are just signs, i.e., indicating negative or positive sign (elements from the set $\{-1,1\}$).