Applications create custom node classes to represent object properties, and then add objects of those classes to a graph. The base class for graph nodes is AcDbEvalExpr. This class exposes a virtual evaluate() function that is called on each activated node when the graph is evaluated. Typically, an application derives node classes from AcDbEvalExpr and implements at least the evaluate() and value() functions.
AcDbEvalExpr also provides a number of virtual notification callback functions that let the application define how its objects respond to various graph events. AcDbEvalExpr inherits persistence from its AcDbObject base class.