The Event Notification Service (ENS) broadcasts several types of events. On order events there are both a FillAmount and FilledAmount field. The first informs how much was filled in that single event and the later how much of the totality is filled. For example if you receive an event like this:
{
"ReferenceId": "U_939",
"Timestamp": "2018-07-11T13:39:30.239056Z",
"Data": [
{
"ActivityType": "Orders",
"Amount": 10,
..
"FillAmount": 4,
"FilledAmount": 10,
..
}
]
}
This would indicate that 4 units was filled in that event and that the total filled amount is now 10. If the orders was on a total of 10 units the order would now be considered filled.
More Resources
Learn page - Event Notification