There are many ways to set permissions in order to determine whether a user can execute an action with an item. This can be denied or granted to specific users or roles.
More on this in the doc on Permissions: http://docs.sitefinity.com/overview-permissions
It is however not clear to me what exactly you're asking regarding Site Sync. Can you elaborate more in details on what exactly you're trying to achieve, maybe with an exact use case scenario?
Please note that the user who has permissions to access the Site Sync module from the backend is allowed to synchronize all the content that is updated and waiting to be synced. Here is a documentation article about the content that can be synchronized from the source to the target server: http://docs.sitefinity.com/synchronized-content
Can you please provide more details if you would like to allow a particular user to be able to sync only particular content to the target?
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