Server is the role of an application or service that publishes content for clients to consume. The server expects one or more client to sync with it. The server does not store sync state for each client to achieve higher performance and scalability.
Client is the role of an application that consumes content published by a given server. The client stores the state of sync. Client initiates the sync and decides what tables to synchronize and the sync direction.
If you are familiar with SQL Merge Replication, server is a “publisher” and client is a “pull subscriber”
Thanks!