They give addition information for current class

There are several type of class attributes:

  1. Group Attribute
     ... [Toolbox("General")]
    public class MyComponent{....} ...
    The component can be find in "General" group of components
    Adding MyComponent

     Default Group is General. [Toolbox("General")]

  2. Parent Attribute
    public class Image ... [Parent("MyParent")] 
    public class MyComponent {....} ...
  3. Child Attribute

      ... [Child("MyChild")]
    public class MyComponent{....} ...