Development Objects in packages are, by default, not visible to objects in other packages - and therefore are not usable. In order to make an object visible to other packages, it must be exposed in one or more package interfaces. Such objects are called Exposed Objects / Visible elements.
Objects in other packages can use an object only if it is exposed in one or more package interfaces.
The level of usage of an exposed object by a client package is determined by the scoped usage types attached with the object in the said interface. A scoped usage type is a usage type with a flag "Allowed for Propagation". If this flag is set, the client package is allowed to expose in its interface, otherwise it is only allowed to use it.
An exposed object can only be added by specifying the object in Exposed Objects List along with the scoped usage types. The objects that are to be added to a package interface should be
Or
You have now added one or more Development Objects to a package interface. These elements are now visible - and can be made available - to other packages.