Why does app:collection contain multiple app:categories?

[John Stracke <francis@xxxxxxxxxxxx>]

In 5023, section 8.3.3, it's specified that an <app:collection> element 
MAY contain any number of <app:categories> elements.  Why? An 
<app:categories> is already a list; what does it mean if we have more 
than one?

It's worth noting that 8.3.6 seems to think that there should be only 
one <app:categories>: "The "app:categories" element provides a list of 
the categories that can be applied to the members of a Collection."

My apologies if this has already been discussed; I did Google the 
mailing list archive first.