When you connect primary sites to form a hierarchy, one site is the parent site, and the other site is the child site. You connect sites by changing a site's parent; that is, for each site that you want to make into a child of site A, you set its parent site to be site A. If there will be five children, you do this five times, once on each child's site server.
You can connect primary sites to form any kind of hierarchy you prefer, but no site can have more than one parent, and sites cannot be connected circularly (meaning a site cannot be its own subsite).
A primary site that has no parent is a central site. Every SMS system must have a central site.
Note Do not confuse the terms child site and secondary site:
Change a Site's Parent
About Site Hierarchies
Configuring SMS Site Settings Overview