Dimension or Degenerate Dimension?
These are the hints:


The dimension consists only of natural key (e.g. invoice's or order's number) and contains no other attributes (because they've been placed in dimensions of their own). 
The dimension table grows proportionately with the fact table, e.g. every time you insert new order lines into the fact table, you have to insert new OrderNumber dimension rows. 
You never use the dimension to filter data when populating a report and it makes no sense to use its values as a row header. 


Read Design Tip #46: Another Look At Degenerate Dimensions from the Kimball Group