Definition: A conversion interaction in which a set of physical entities, at least one being a macromolecule (e.g. protein, RNA, DNA), aggregate via non-covalent interactions. One of the participants of a complexAssembly must be an instance of the class complex (via a physicalEntityParticipant instance). Comment: This class is also used to represent complex disassembly. The assembly or disassembly of a complex is often a spontaneous process, in which case the direction of the complexAssembly (toward either assembly or disassembly) should be specified via the SPONTANEOUS property. Synonyms: aggregation, complex formation Examples: Assembly of the TFB2 and TFB3 proteins into the TFIIH complex, and assembly of the ribosome through aggregation of its subunits. Note: The following are not examples of complex assembly: Covalent phosphorylation of a protein (this is a biochemicalReaction); the TFIIH complex itself (this is an instance of the complex class, not the complexAssembly class).
Source:http://www.biopax.org/release/biopax-level2.owl#complexAssembly