A collection description is the formalization combined with the Pattern Expression, that documents the used patterns and the rationalizations upon which the collection is based.

So the abstraction layers are:

Pattern expression

Write your pattern expression here

Collection description

Sum up the elements in the collection here

Technical

Be concrete and go into detail of things that require it, especially parts of the collection that is complex or of a special nature.

Consider using a drawing to illustrate relations or for instance include code/XML snippets