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The Sitecore Scheduler fires on the CM server to initiate the sync. Salesforce Leads and Contacts are synchronized and S4S EXM uses a date/time in the Sitecore Content tree to determine the last time sync occurred (see image below). This is to limit the time required to undertake complete the sync and prevent unnecessarily unnecessary updating of the same data. When a sync has been completed, the date/time value is updated on the CM server and the same item is also published to the CD server.
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