UML Visibility

One object must be able to see another object in order to use it. Ways to establish visibility of one object from another are:

Parameter visibility - Pass a parameter reference to a method in the object. While that method is active, the object specified by the parameter is visible. (temporary visibility)
Attribute visibility - Use an object attribute as a reference to the other object. (permanent visibility)
Global - Have the object reference be globally visible. (permanent visibility)
Local - Have the object reference be locally visible. Acquire an object reference through another object. Active only while the method is being run. (temporary visibility)