Definition: A step in an ordered pathway. Rationale: Some pathways can have a temporal order. For example, if the pathway boundaries are based on a perturbation phenotype link, the pathway might start with the perturbing agent and end at gene expression leading to the observed changes. Pathway steps can represent directed compound graphs. Usage: Multiple interactions may occur in a pathway step, each should be listed in the stepProcess property. Order relationships between pathway steps may be established with the nextStep slot. If the reaction contained in the step is a reversible biochemical reaction but physiologically has a direction in the context of this pathway, use the subclass BiochemicalPathwayStep. Example: A metabolic pathway may contain a pathway step composed of one biochemical reaction (BR1) and one catalysis (CAT1) instance, where CAT1 describes the catalysis of BR1. The M phase of the cell cycle, defined as a pathway, precedes the G1 phase, also defined as a pathway.
Source:http://www.biopax.org/release/biopax-level3.owl#PathwayStep