Config The OCG::Config object represents an optional-content (OC) configuration structure (see section 4.10.3 in PDF Reference), used to persist a set of visibility states and other optional-content information in a PDF file A document has a default configuration, saved in the D entry in the 'OCProperties' dictionary (that can be obtained using pdfdoc.GetOCConfig()), and can have a list of other configurations, saved as an array in the 'Configs' entry in the OCProperties dictionary. 

The OCG::Context object represents an optional-content context in a document, within which document objects such as words or annotations are visible or hidden. 

Group The OCG::Group object represents an optional-content group. 
OCMD The OCMD object represents an Optional Content Membership Dictionary (OCMD) that allows the visibility of optional content to depend on the states in a set of optional-content groups (OCG::Group).