I can not find any describtion of a XC9 Sales Activity. What is a XC9 Sales Activity exactly?
I got a quick and elaborate answer from Rob Earlam on Slack:
"I asked the same question to the Product team a while back and got this response.......":
A SalesActivity represents an instance of a charge or credit that occurs related to an Order.
When you provide your payment information and place the order, it is often the case that the actual charge to your payment instrument doesn't occur right away. We return back the Order confirmation and process the payment asynchronously (after validating in the PendingOrders minion, that the item can actually be delivered).
This may be due to buying a backorder or preorder or some other situation (fraud alert, inventory issue, etc) which causes you to delay settling the charge. When the order is settled, a SalesActivity is generated, representing that charge.
It is also often the case where multiple SalesActivities are generated during the lifecycle of an Order. For example, if the order is split and charges are done at two different times, a SalesActivity is generated, when each Order piece is fulfilled.
It is also possible to have a negative SalesActivity. If an Order is placed and then the customer subsequently returns all of some of the items in the order, when the customer is refunded, a SalesActivity with a negative number is generated.
These can be thought of as Debits and Credits in a Ledger and represents the history of charges/credits related to a specific order. The sum of the SalesActivities should match the GrandTotal of the Order."