Note 4. Revenue

To reflect the organization of our business operations, we divide revenue into two categories: revenue from sales of new systems and revenue arising from the sale of used systems, parts and labor to customers who own systems, which we refer to as “Aftermarket.”

Revenue by categories used by management are as follows:


	Three months ended
	March 31,
	2020		2019
	(in thousands)
Systems	$	82,338	$	57,092
Aftermarket		36,653		34,385
	$	118,991	$	91,477

We also consider revenue by geography. Revenue is allocated to geographic markets based upon the location to which our products are shipped and in which our services are performed. Revenue in our principal geographic markets is as follows:


	Three months ended
	March 31,
	2020		2019
	(in thousands)
North America	$	10,661	$	11,607
Asia Pacific		96,828		62,927
Europe		11,502		16,943
	$	118,991	$	91,477

Our system sales revenue transactions give rise to contract liabilities (in the case of pre-payments and the fair value of goods and services to be delivered after the system delivery, such as installation and certain warranty obligations).

Contract liabilities are as follows:

Contract liabilities are reflected as deferred revenue on the consolidated balance sheet and relate to payments invoiced or received in advance of completion of performance obligations under a contract. Contract liabilities are recognized as revenue upon the fulfillment of performance obligations.


	Three months ended
	March 31,
	2020		2019
	(in thousands)
Balance, beginning of the period	$	29,251	$	22,584
Deferral of revenue		9,799		4,078
Recognition of deferred revenue		(4,944)		(6,181)
Balance, ending of the period	$	34,106	$	20,481

The majority of our system transactions have payment terms of 90% due upon shipment of the tool and 10% due upon acceptance. Aftermarket transaction payment terms usually provide that payment is due either within 30 or 60 days after the service is provided or parts delivered.

December 31,

(in thousands)

Contract liabilities