A partner url is a sub-section of a site, in which there could be hundreds or thousands of partner urls under one domain, whereas the domain is the main site, and there is only one.  For example:

A domain may be:

• www.bigsite.com
  

and the partner urls may be:

• www.bigsite.com/department
  
• www.bigsite.com/department/categories
  
• www.bigsite.com/products.
  
• www.bigsite.com/policies
  
• www.bigsite.com/orders
  
• www.bigsite.com/products/forkids
• www.bigsite.com/products/forkids/girls