Each value of this property represents the stoichiometric coefficient for one of the physical entities in an interaction or complex. For a given interaction, the stoichiometry should always be used where possible instead of representing the number of participants with separate instances of each participant. If there are three ATP molecules, one ATP molecule should be represented as a participant and the stoichiometry should be set to 3.
Source:http://www.biopax.org/release/biopax-level2.owl#STOICHIOMETRIC-COEFFICIENT