Collections may be grouped together into more collections. Data vs. metadata . Any collection description mechanism will have to differentiate between these two. What kind of attributes are needed . Useful to agree on some terminology.
Collection is a grouping of items.
May be nested. Application is an access protocol for the collection. Service provider is the organisation that provides access to the collection. AnP also wonders if it is useful to distinguish search, browse and navigate, where browse implies browsing within a collection while navigate implies browsing up and down a heirarchy of collections. All you can do you can do with level is moving up and down within, but you can not move levels within distinguished collections.