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 binomial PIVP with degree at most 2 ) for restricting ourselves to at most bimolecular reactions.