An Express relation is a link between one or many dimensions and another one. You can set up as many relations as needed between any number of dimensions. It is an extraordinary powerful tool in Express, allowing users to drill down or up along dimensions. This is a very simple hierarchy. You can have as many levels as needed.
Some links can 'skip' levels.
You may have many hierarchies defined on a single dimension. The above figure tries to illustrate how to drill down along independent attributes. Assume you sell chocolates. The color attribute can be White, Black, Brown. The weight attribute can be 100gr, 200gr, 300gr.