Quarterly report pursuant to Section 13 or 15(d)


9 Months Ended
Sep. 30, 2021
Revenue from Contract with Customer [Abstract]  
Revenue Revenue
    Unbilled accounts receivable (“AR”) for the periods presented primarily represent the difference between revenue recognized based on the relative selling price of the related performance obligations and the contractual billing terms in the arrangements. Deferred revenue for the periods presented primarily relates to extended service contracts, installation, and training, for which the service fees are billed in advance. The associated deferred revenue is generally recognized ratably over the extended service period or when installation and training are complete.
The following table summarizes the changes in the unbilled AR and deferred revenue balances for the nine months ended September 30, 2021 (in thousands):
Unbilled AR, December 31, 2020 $ 1,925 
Additions 145 
Transferred to trade receivable (350)
Unbilled AR, September 30, 2021 $ 1,720 
Deferred revenue, December 31, 2020 $ 25,723 
Additions 22,089 
Revenue recognized (17,535)
Deferred revenue, September 30, 2021 $ 30,277 
    At September 30, 2021, the short-term portion of deferred revenue of $25.7 million and the long-term portion of $4.6 million are included in deferred revenue and other long-term liabilities, respectively, in the consolidated balance sheet. As of September 30, 2021, we expect to recognize revenue associated with deferred revenue of approximately $10.5 million in 2021, $15.9 million in 2022, $2.1 million in 2023, $1.2 million in 2024, and $0.6 million thereafter.