It is possible to configure which projects are synchronized, i.e., select all or only a particular subset of the source server projects to be synchronized to the target.
The target server specifies the user/password of the source server when the target server initiates the synchronization process. The synchronization process then takes all accessible projects from the source server. The source server administrator can control which projects are accessible by managing user permissions on his server. If the specified synchronization user has permission to read all the projects on the source server, all projects will be synchronized during the process. If the specified user only has permissions to access a particular subset of various projects, only these projects shall be synchronized during the process.
It is highly advisable to create a special dedicated user on the source server for synchronization purposes and not reuse the real existing user accounts. Only read permissions should be given to this account.
Note that different synchronized project sets can be specified on the source for each different target. Just use different users for each target and define the project set as described above.