Defines the physicalEntity subunits of this complex. This property should not contain other complexes, i.e. it should always be a flat representation of the complex. For example, if two protein complexes join to form a single larger complex via a complex assembly interaction, the COMPONENTS of the new complex should be the individual proteins of the smaller complexes, not the two smaller complexes themselves. Exceptions are black-box complexes (i.e. complexes in which the COMPONENTS property is empty), which may be used as COMPONENTS of other complexes (via a physicalEntityParticipant instance) because their constituent parts are unknown / unspecified. The reason for keeping complexes flat is to signify that there is no information stored in the way complexes are nested, such as assembly order.
Source:http://www.biopax.org/release/biopax-level2.owl#COMPONENTS