Global collections
Besides part and association characteristics, which may define object instance collections, a number of global collections not defined in the context of an object type might be important within the subject area. Collections are relevant sets of object instances, that can be derived or that really do exist.
When defining collections, it does not matter, whether collections do exist as such or have to be calculated. It is the conceptual view to a set of objects as collection, which counts.
One may define a collection of persons, who had ever been in Brlin. When "having been in Berlin" is not a person attribute, one may still collect all those persons in a collection.