This property stores the members of a generic physical entity. For representing homology generics a better way is to use generic entity references and generic features. However not all generic logic can be captured by this, such as complex generics or rare cases where feature cardinality is variable. Usages of this property should be limited to such cases.
Source:http://www.biopax.org/release/biopax-level3.owl#memberPhysicalEntity