Dummies and point clouds, since point-based, can not be detected by ray-type or randomized-type proximity sensors.
Detectable objects can be detected by all proximity sensors, or only by specific types of proximity sensors, or a sub-category of proximity sensors as listed:
Collections are also detectable, since they might contain detectable objects.
Detectable objects can have their detectable property individually enabled or disabled, and this for all types of proximity sensors (enabled by default for non-pure shapes). This can be set in the object common properties or through the simSetObjectSpecialProperty API function.
Additionally, detectable objects can have their detectable property overridden depending on their related model properties (if they are part of a model). Refer to the model dialog for more information.