Our revenue is generated primarily from the sale of products and services. Product revenue primarily consists of sales of our instruments and related consumables; service and other revenue consist primarily of revenue earned from product maintenance agreements.
We account for a contract with a customer when there is a legally enforceable contract between us and the customer, the rights of the parties are identified, the contract has commercial substance, and collectability of the contract consideration is probable. Revenues are recognized when control of the promised goods is transferred to our customers, or services are performed, in an amount that reflects the consideration we expect to be entitled to in exchange for those goods or services. Invoicing typically occurs upon shipment, or delivery in the case of an instrument, and payment is typically due within 30 days from invoice. In instances where the right to payment or transfer of title is contingent upon customer acceptance of the product, revenue is deferred until the acceptance criteria has been met. Revenue from instrument service contracts is recognized as the services are rendered, typically evenly over the contract term. Revenue from development agreements generally includes upfront and milestone payments. Revenue for these agreements is recognized when each separate performance obligation is satisfied.
We may enter into, or periodically modify, contracts with customers that include a combination of promised products and services, resulting in arrangements containing multiple performance obligations. We determine whether each product or service is distinct, in order to identify the performance obligations in the contract and allocate the contract transaction price among the separate performance obligations. A product or service is considered distinct from other obligations in a contract when it provides a benefit to the customer either on its own or together with other resources that are readily available to the customer and is separately identified in the contract. We consider a performance obligation satisfied once we have transferred control of a good or service to the customer, meaning the customer has the ability to use and obtain the benefit of the good or service. Therefore, instrument revenue is recognized upon transfer of control of the asset to the customer, which is generally upon delivery for sales made to our non-distributor customers and upon shipment for sales made to our distributor customers.
The consideration for contracts with multiple performance obligations is allocated between separate performance obligations based on their individual standalone selling price. We determine the best estimate of standalone selling price primarily using historical average selling prices combined with an assessment of current market conditions. If the standalone selling price is not directly observable, we rely on estimates by considering multiple factors including, but not limited to, overall market conditions, including geographic or regional specific factors, internal costs, profit objectives, pricing practices, and other observable inputs. We recognize revenues as performance obligations are satisfied by transferring control of the product or service to the customer or over the term of a product maintenance agreement with a customer. Our revenue arrangements generally do not provide a right of return. Revenue is recorded net of discounts and sales taxes collected on behalf of governmental authorities. We update the transaction price for expected consideration, subject to constraint. Where we expect, at contract inception, the timing of payments to be consistent with the transfer of goods or services or the contract duration to be one year or less, we do not adjust the transaction price for the effects of a significant financing component.
Modification of existing contracts with customers could change the scope or the price of the contract, or both. When a contract modification occurs, we exercise judgment to determine if the modification should be accounted for as: (i) a separate contract, (ii) the termination of the original contract and creation of a new contract, (iii) a cumulative catch-up adjustment to the original contract, or a combination thereof. Further, contract modifications require the identification and evaluation of the performance obligations of the modified contract, allocation of revenue to the remaining performance obligations and determination of the period of recognition for each identified performance obligation.
Certain of our agreements provide options to customers which can be exercised at a future date, such as the option to purchase our product at discounted prices, among others. In accounting for customer options, we determine whether an option is a material right and this may require us to exercise judgment. If a contract provides the customer an option to acquire additional goods or services at a discount that exceeds the range of discounts that we typically give for that product or service for the same class of customer, or if the option provides the customer certain additional goods or services for free, the option is considered a material right and, therefore, a performance obligation. If the standalone selling price of the option is not directly observable, an estimated standalone selling price is utilized which considers adjustments for discounts that the customer could receive without exercising the option and the likelihood that the option will be exercised.
Additionally, we generally provide a one-year warranty on instruments. We accrue the cost of the assurance warranty when revenue of the instrument is recognized. Employee sales commissions are generally recorded as selling, general, and administrative expense when incurred as the amortization period for such costs, if capitalized, would have been one year or less.