The components of net deferred income tax assets for income tax purposes are as follows (in thousands):
The net deferred tax assets as of February 28, 2018 in the above table include the deferred tax assets of our Italian and Canadian subsidiaries amounting to $7.4 million and $7.6 million, respectively, which were disclosed narratively in the fiscal 2018 Form 10-K. The deferred tax assets primarily relate to net operating losses (NOL’s) and research and development expenditure pool carryforwards. We had provided a 100% valuation allowance against these deferred tax assets at February 28, 2018, as it was more likely than not that the deferred tax assets would not be realized.
As of February 28, 2019 and 2018, we maintained a valuation allowance with respect to certain of our deferred tax assets relating primarily to NOL’s in certain non-U.S. jurisdictions and certain state tax credits that we believe are not likely to be realized. During fiscal 2019, we decreased the valuation allowance against our deferred tax assets by approximately $5.9 million, as it is more likely than not that these deferred tax assets would be realized based upon the assessment of positive and negative evidence. This reduction in our valuation allowance is primarily attributable to a release of valuation allowance against foreign deferred tax assets, partially offset by an increase in valuation allowances for state tax credits.
At February 28, 2019, we had net operating loss carryforwards of approximately $30.1 million, $60.8 million and $44.7 million for federal, state and foreign purposes, respectively, expiring at various dates through fiscal 2039. Approximately $18.3 million of foreign net operating loss carryforwards do not expire. The federal net operating loss carryforwards are subject to various limitations under Section 382 of the Internal Revenue Code. If substantial changes in our ownership were to occur, there may be certain annual limitations on the amount of the NOL carryforwards that can be utilized
As of February 28, 2019, we had R&D tax credit carryforwards of $9.1 million and $8.9 million for federal and state income tax purposes, respectively. The federal R&D tax credits expire at various dates through 2039. A substantial portion of the state R&D tax credits have no expiration date.
We adopted the updated guidance on stock based compensation and we have tax deductions on exercised stock options and vested restricted stock awards that exceed stock compensation expense amounts recognized for financial reporting purposes. The gross excess tax deductions were $2.9 million, $2.6 and $0 in fiscal years 2019, 2018 and 2017, respectively. Under the new guidance, all excess tax benefits and tax deficiencies are recognized in the income statement as they occur.
We follow ASC Topic 740, “Income Taxes,” which clarifies the accounting for income taxes by prescribing a minimum recognition threshold that a tax position is required to meet before being recognized in the financial statements. Management determined based on our evaluation of our income tax positions that we have uncertain tax benefit of $3.2 million, $1.0 million and $1.0 million on at February 28, 2019, 2018 and 2017, respectively, for which we have not yet recognized an income tax benefit for financial reporting purposes.
At February 28, 2019, we increased the uncertain tax benefits related to certain foreign net operating loss carryforwards. Such deferred tax assets were previously offset by a valuation allowance so that the increase in the unrecognized tax benefit coupled with the reduction of the valuation allowance on such net operating losses did not result in an income tax expense during the current fiscal year. If total uncertain tax benefits were realized in a future period, it would result in a tax benefit of $3.2 million. As of February 28, 2019, our liabilities for uncertain tax benefits were netted against our deferred tax assets on our consolidated balance sheet. It is reasonably possible the amount of unrecognized tax benefits could be reduced within the next 12 months by at least $0.6 million.
We recognize interest and/or penalties related to uncertain tax positions in income tax expense. No amounts of interest and/or penalties have been accrued as of February 28, 2019.
Table:
 |  | February 28,
 | 2019 | 2018
Net operating loss carryforwards | $19,269 | $22,013
Depreciation, amortization and impairments | (11,945) | (11,112)
Research and development credits | 19,189 | 17,432
Stock-based compensation | 2,783 | 2,376
Other tax credits | 1,018 | 2,015
Inventory reserve | 624 | 292
Warranty reserve | 313 | 429
Payroll and employee benefit accruals | 2,220 | 1,941
Allowance for doubtful accounts | 454 | 354
Other accrued liabilities | 6,208 | 8,975
Convertible debt | (10,822) | (194)
Other, net | 3,281 | 3,904
Gross deferred tax assets | 32,592 | 48,425
Valuation allowance | (10,929) | (16,844)
Net deferred tax assets | $21,663 | $31,581
Reported as: |  | 
Deferred tax assets | $22,626 | $31,581
Deferred tax liabilities | (963) | -
Net deferred tax assets | $21,663 | $31,581


Question: What does the deferred tax assets primarily relate to?
Answer: net operating losses (NOL’s) and research and development expenditure pool carryforwards