The taxable amount of your mother's 1099-R distributions depends on the details in Boxes 1, 2a, and 5. If Box 2a is marked "UNKNOWN," you may need to calculate the taxable portion. This involves reviewing the account's history to distinguish between pre-tax and after-tax contributions. For example, if the distribution includes after-tax contributions, that portion isn't taxable. If you choose to use the simplified method to calculate the taxable amount, the calculation for this is in the instructions for Form 1040.
If Box 5 is marked "NONE," it means there were no employee contributions or insurance premiums paid with after-tax dollars. This box is used to report any amounts that aren't taxable because they represent a return of your own contributions
If Box 9b is marked "UNKNOWN," it means the payer couldn't determine the total amount of employee contributions
IRS - Simplified Method Worksheet
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