And they already know their total loss?

Read the IRS guidance, such as:

https://www.irs.gov/newsroom/irs-announces-tax-relief-for-texas-severe-winter-storm-victims

"Casualty Losses

Affected taxpayers in a federally declared disaster area have the option of claiming disaster-related casualty losses on their federal income tax return for either the year in which the event occurred, or the prior year. See Publication 547 for details.

Individuals may deduct personal property losses that are not covered by insurance or other reimbursements. For details, see Form 4684, Casualties and Thefts PDF and its Instructions. PDF

Affected taxpayers claiming the disaster loss on a 2020 return should put the Disaster Designation, "Texas – Severe Winter Storms," in bold letters at the top of the form. Be sure to include the disaster declaration number, FEMA 4586, on any return. See Publication 547 for details."