A leader is an entity that consists of a polyline or spline, arrowhead at one end, and an annotation at the other end. The annotation can be an OdDbMText object, OdDbFcf object which is a Frame Control Feature (tolerance), or a block reference.
Overview of Leaders
Specific Properties of Leaders
Working with Leader Lines
Working with Annotations of Leaders
Example of Working with Leaders
OdDbLeader Class in API Reference
A sample command for working with leaders is located in the Drawing\Examples\DevGuideCommands subdirectory of the ODA installation folder.