SCCM Boundary Group Question

I only have only one site system server here at the main office but I have several branch offices where I'm making a Distribution Point server at each. When creating a boundary group for a DP, I am confused by the Reference tab. I check the box that says "use this boundary group for site assignment" but at the bottom where it says to add a Site System Server, do I add the Site System server at the main office or do I add the DP server?

Thank you,
ex.PNG