Nested Collections
Normal collections, as well as taxonomy collections, can be nested arbitrarily. Any photo contained in a sub collection will then automatically be part of the enclosing collection, too. Creating sub collections can be achieved in two ways:
Right-clicking a collection in the collection pane will bring up a context menu that allows to directly create a new sub collection.
An existing collection can be dragged into another collection within the collection pane. This way, a global collection can be made into a sub collection.
Collection hierarcies, similarly to hierarchical keywords, enable the creation of refined categorization systems in addition to normal album-like flat collections.