Hello again Olivier,

"Nestings" is a connector that shows nesting (containment) relationships in the diagrams below in Astah.
- Class Diagram
- UseCase Diagram
- Component Diagram
- Composite Structure Diagram
- Requirement Diagram

Nesting shows the source model is nested within the target model. In the figure below, Inner Class is nested within Class.










In Astah, it automatically shows the nested relationship hierarchy in the structure tree as below.


















Here is the example of drawing a class which has multiple inner classes.

















Related resource:
UML Superstructure Specification, v2.4.1
P.48 - 7.3.7 Class - Under Associations:
P.109 - 7.3.38 Package

Cheers,
SJ