Supply ID is the feed
Site ID is the website or placement

Our Traffic is broken up by Supply and Site ID.  At the bottom level, you have the Site ID (the website that collected the Push subscriber). These are grouped in to supply each with an ID as shown below.

If you are blacklisting or whitelisting a Supply ID, it will affect all of the supply sources under it. Whereas blacklisting or whitelisting a Site ID won't affect other sites under that supply.

*Site ID is a combination of Supply ID and Source (our old macro for Placements and websites). Source was not unique and needed to be attached to the Supply ID so we created Site ID to make it a little more simple (meaning you don't have to attach Supply ID because it is already apart of the Macro)*