[jmull]

In this case, client means your mac running macOS — there will be a system update process running on your mac.

The server is the the apple server the update process talks to to figure out what to download (and then to download it).

(I haven’t looked at it but there are probably multiple mac processes, with their own set of relationships, and multiple Apple services involved, but for the purpose of this issue, you can think of the client side being anything running on your mac, and server side as anything running on an Apple server.)