It’s the number of unit sold that day. The history endpoint is what you would see looking at the table history view of a given type in game. Where volume maps to in game quantity.
No, the orders are the number of orders that were filled. It is different than the volume as a single order can have multiple items in it.
The orders is basically the number of transactions that day, and the volume/quantity is the number of items that changed hands in all of those transactions.