Hierarchies are usually short-term representations, as the relations between values at the top are often subject to change. Tagging is a much more flexible way

Hierarchies are usually short-term representations, as the relations between values at the top are often subject to change. Tagging is a much more flexible way of linking between ideas, and if you really want to, you can create a hierarchy of tags. How tags are grouped is subject to preference too, but bottom-up definitions seem to allow more flexibility than top-down ones like in hierarchies.