I am currently testing the setup. How do I synchronize old data? Customers, Inventory, PO, SO, etc?
Synchronisation of old data (prior to dsync setup) would be entirely separate configuration.
The standard procedure for this is to configure the one-off synchronization links and run those first. After that's done, you would then use the 'Synchronise' on real-time/ongoing synchronization setup to fetch all the records from connected systems and pair them. This pairing requires shared keys to be selected on both endpoints.
Shared key is a field which pairs the same record across multiple systems (customer email for example). It must be correctly mapped and all records in each system must have a value entered in that shared key field. (eg. if you set the shared key to be email address and if your customer does not have an email address defined, the pairing fails).
Our standard setup which was performed does not do this. It transfers data on near realtime basis - only the new/updated records after the links were established.