This is the number of transactions a single engine settles within a twelve month period.

This twelve month period is actually calculated as a moving window of time and is accounted for each day.

throughput = ((todays total)+(last 364 days total))

Example:
A merchant authorizes fifty transactions in a day. Prior to nightly settlement the merchant voids ten transactions. The merchant settles forty transactions which count towards the annual total.

Now if this merchant were to authorize and settle one thousand transactions on Jan 01, and not use the engine again until Jan 02 of the next year, the engines throughput would be starting again at zero.

Note: The engine maintains its own internal throughput counter.