not knowing for sure what you are wanting I would say Export Scenario’s.
You can export to a CSV or other formats and I believe you can schedule to sync those to FTP. not 100% on SFTP but think that is possible too.
https://openuni.acumatica.com//pdf/S125_Document_Management(V4.20).pdf
page 18
As Shawn said, File Synchronization can use SFTP but you have to generate a certificate and package that up so you can upload it into Acumatica for use in the File Synch process. I found that process challenging to figure out - but it works.
Generating the CSV or flat file might just be an Export Scenario depending on the complexity of the layout. If it is too complex, then you’ll want to consider creating your own Data Provider.
Figuring out what you last sent is a little trickier. You’ll need a way of limiting the output to only new transactions. Business Events and Push Notifications make use of temp tables to allow them to react to new/modified/deleted transactions - you might be able to leverage those.
Otherwise, part of me would be tempted to have an Exported flag on your transactions that is toggled when they’ve been packaged up - but that’s a little more complicated.
if you need full, New/Modified/Deleted you might prefer to use the webhook. Assuming your target can receive them, that process can push the update data to the webhook endpoint in realtime so you updates as they happen.