Collidable objects are objects that can be tested for collision against other collidable objects, i.e. that will register a collision state. This does not mean that they will respond to collision (i.e. respondable), which is something different. Collidable objects include:
Dummies and point clouds, since point-based, can only collide against OC trees (which are volume-based).
Collections are also collidable, since they might contain collidable objects.
Collidable objects can have their collidable property individually enabled or disabled (enabled by default for non-pure shapes, OC trees and point clouds). This can be set in the object common properties or through the sim.setObjectSpecialProperty API function.
Additionally, collidable objects can have their collidable property overridden depending on their related model properties (if they are part of a model). Refer to the model dialog for more information.