Before revision:  It uses an encoding of real variables by two non-negative variables for concentrations, and a transformation to an equivalent binomial PIVP with degree at most 2 for restricting to at most bimolecular reactions.
After revision:  It uses an encoding of real variables by two non-negative variables for concentrations, and a transformation to an equivalent quadratic PIVP (i.e. with degrees at most 2 for restricting to at most bimolecular reactions.