Compound objects are comprised of more than one object. For example, a screen has its own definition, which includes one or more screen tables, a report has one or more report tables, and an object set has a variety of component objects.
You can obtain the promotion rights to a compound object, but holding the right to a compound object is not the same as having the rights to the individual components. If you have the promotion rights to a compound object, you hold the rights to the compound object definition. However, this does not give you the rights to the child objects that make up that compound object. You must still obtain these rights separately.