You need two pieces of info: Tax Year and Disaster Year.

Disaster Year 2020 is the transition. The transition is Tax Year 2020 for Disaster Year 2020 8915-E usage ends; it is used only this one year. Then, Tax Year 2021 for Disaster Year 2020 is the cut-over year and Forever form 8915-F begins.

For Tax Year 2021 and later, and Disaster Year 2020 or later, the "forever" form to use is -F.

For any Tax Year, for Disaster Years 2019 and earlier, use the year-specific Disaster Year form (tax- and disaster- specific), such as -D is 2019 Disasters and always will be. Eventually, the claim window for 2019 disasters and distribution reporting and repayment reporting will Close. That means tax year filing of 2021 for a disaster from 2019 will be filed using the 2021 Tax Year form 8915-D.

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