A compound is a feature class type that consists of a logical group of feature classes, which include a parent and a child feature class.
Compounds are used for polygon or line string objects where particular line segments can have different attributes. Therefore a compound represents a group of line segments as one object. For example, several child line segments are combined as a parent polyline or several child line segments are combined as parent polygon.
Data Model: Compound feature classes
When you digitize features of a compound feature class, you digitize the child line strings, and the parent polygon is generated.