Note 4. Revenue

To reflect the organization of our business operations, we divide revenue into two categories: revenue from sales of ion implant 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,
	2019		2018
	(in thousands)
Ion Implant	$	57,092	$	85,475
Aftermarket		34,385		36,710
	$	91,477	$	122,185

Revenue by geographic markets is determined based upon the location of where our products are shipped and our services are performed. Revenue in our principal geographic markets is as follows:


	Three months ended
	March 31,
	2019		2018
	(in thousands)
North America	$	11,607	$	10,523
Asia Pacific		62,927		97,689
Europe		16,943		13,973
	$	91,477	$	122,185

Contract assets and contract liabilities are as follows:


	March 31,		December 31,
	2019		2018
	(in thousands)
Contract assets	$	—	$	—

Contract liabilities	$	20,481	$	22,584

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,
	2019		2018
	(in thousands)
Revenue recognized in the period from:
Amounts included in contract liability at the beginning of the period	$	6,181	$	6,342
Performance obligations satisfied in previous periods	$	—	$	—

The majority of our system transactions have payment terms of 90% due upon shipment of the tool and 10% due upon installation. Aftermarket transaction payment terms are such that payment is due either within 30 or 60 days of service provided or delivery of parts.