From OpenCog

The ConnectorDirNode is a type of LexicalNode used to in the mating of some kinds of Connectors. It is intended to indicate connector polarity, e.g. when connectors have a to/from, head/dependent, parent/child, male/female, +/- type of polarity that is associated with them.

ConnectorDir's need not be bipolar; there might be more than two kinds of ConnectorDir, with specific mating rules.

See Connectors and Sections for details.