How to use Multiple DFF Attribute Categories on an Invoice

Trying to figure out how the DFFs work with Attribute Categories.  For an invoice distribution line we have three attribute categories defined.  But it seems we can only populate one category's fields.  When we select a second category and fill out the fields and hit save, then the first categories  data is removed.  Is this intended functionality?  What's the use of having multiple categories if you can only use one per invoice?  What am I missing?