Currently Sage 200 Online does not have Archiving and Purging available in Sales and Purchase Ledger and in SOP and POP. This means that the customer either has to pay more for database storage or the BP has to download their data into an On-Premise version, archive and purge in here, then upload the data back into the Online version, which is likely to be chargeable too. The Purge archived orders menu option is visible in the Online SL and PL menus, which is confusing too. Obviously this option can be removed from the menus in System Administration, but if you cant do purging then the option shouldnt be there at all.
Sage 200 Extra Online is no longer available. Sage 200 Professional and Sage 200 Partner Cloud does already contain the functionality mentioned.