Table:
|  | Years ended December 31, |  |
| --- | --- | --- |
|  | 2019 | 2018 |
| Deferred license fees revenues | $28 | $- |
| Deferred NRE revenues | 20 | - |
| Deferred AirBar revenues | 6 | 59 |
| Deferred sensor modules revenues | 13 | 16 |
|  | $67 | $75 |
Paragraphs:
Deferred Revenues
Deferred revenues consist primarily of prepayments for license fees, and other products or services for which we have been paid in advance, and earn the revenue when we transfer control of the product or service. Deferred revenues may also include upfront payments for consulting services to be performed in the future, such as non-recurring engineering services.
We defer license fees until we have met all accounting requirements for revenue recognition, which is when a license is made available to a customer and that customer has a right to use the license. Engineering development fee revenues are deferred until engineering services have been completed and accepted by our customers. We defer AirBar and sensor modules revenues until distributors sell the products to their end customers
Under U.S. GAAP, companies may make reasonable aggregations and approximations of returns data to accurately estimate returns. Our AirBar and sensor module returns and warranty experience to date has enabled us to make reasonable returns estimates, which are supported by the fact that our product sales involve homogenous transactions. The reserve for future sales returns is recorded as a reduction of our accounts receivable and revenue and was insignificant as of December 31, 2019 and 2018.
The following table presents our deferred revenues by source (in thousands);
Question:
What do deferred revenues primarily consist of? 
Answer: prepayments for license fees, and other products or services for which we have been paid in advance, and earn the revenue when we transfer control of the product or service