Question about collection-type on reltable or relcolspec elements

Generated links in DITA are determined based on the 
collection type attribute of the container element that 
contains a set of sibling topicrefs.  So, if I set the 
collection-type attribute on the reltable element, is that 
element the "container" for any topicrefs?  

In general the container for topicrefs in a reltable will be 
a relcell element.  The container might also be other topicref, 
topichead, or topicgroup elements within the relcell.  A topicref 
will never be a child of a reltable or of a relcolspec.

If this is true, what is the point of being able to use 
collection-type on the reltable element?  

And for that matter what is the point of being able to use 
collection-type on a relcolspec element?  You can't use 
collection-type on a relrow or a relheader.  You can use it 
on relcell, where it seems to make sense.

paul