All COUNTER 5 reports requested to Library Sales have "Total Item Requests" as the default parameter. This counts the total number of full-text downloads. 

There 3 underlying "events" that count towards this:

1. OA books that have a full-book PDF count as one item request for that ISBN

2. Elements that have a single PDF count as one item request for that ISBN 

3. OA books and non-OA books that have chapters count as one item request for the book ISBN if a user accesses one of those.  

This is per the COUNTER specifics. You can find more information on the direct website or through Counter Metrics page.