Attribute Dimensions and Value Dimensions

Typically, Attribute and Attribute Value dimensions are used to store data in Stage that will not make it to the Cube; stuff like transaction IDs, SKUs, etc, which would be too cumbersome or inefficient to have in UDs. You then look up the data when necessary, in reports or dashboards, using https://onestream.mcoutput.com/1384528/Content/Design%20and%20Reference/Financial%20Model%20Guides/Relational%20Blending%20API.html?Highlight=attribute%20value (which is not BiBlend), either through those APIs or directly with SQL queries over Stage tables. Attributes store text and Attribute Values store numbers.  This is the traditional use for them.

BiBlend augments that feature because it allows you to aggregate on Attribute dimensions, creating custom hierarchies. Suddenly you can get 28 UDs (!), and the world is your oyster. BiBlend also can leverage Attribute Values to speed up calculations, when the input source is already pivoted on Time (i.e. periods are in columns) and you have less than 13 periods.

I believe this is covered in the Foundation Handbook.