Colliders define an approximate shape for an object that’s used by the physics engine to determine collisions with other objects.
2D colliders all have names ending ‘2D’. A collider named without a ‘2D’ ending is a 3D joint. Note that you can’t mix 3D game objects and 2D colliders or 2D game objects and 3D colliders.
The collider types that can be used with Rigidbody 2D are:
See the pages about these components for further details.
USE AUTO MASS:
Note that a Collider’s mass determines the Rigidbody’s mass if you have Use Auto Mass selected. This option is good to use in conjunction with the Buoyancy Effector 2D.
Is something described here not working as you expect it to? It might be a Known Issue. Please check with the Issue Tracker at issuetracker.unity3d.com.