A client will always own at least one site. A site could be considered as a packet of land or a single property of one single address. It's useful to know that a QRID must always belong to a single site and that any site can house an unlimited amount of QRIDs.
You must first create a Client before you can create a Site.
A client can have more than one site. If you have a client with more than one site you do not have to create a new client for each site, you simply add a new site to a client.