I assume that you are referring to box 2a on the form 1099-R.
This box is used if the PAYER can determine the taxable amount (not you, but the retirement plan administrator).
Often the Payer cannot determine the actual Taxable Amount so they leave box 2a blank and may or may not check box 2b.
Why wouldn't a pension or annuity be 100% taxable?
If the taxpayer paid after-tax dollars into the pension or annuity while working, then when the pension or annuity pays out, there is a formula by which it is determined what part of the payout is calculated to be a "return of basis". "Basis" just refers to the amount of after-tax contributions paid into the pension or annuity.
Normally, you would be asked a series of questions about your basis or "contributions" to the pension or annuity. You are expected to have tracked this, although sometimes if you have had the same plan administrator all along, they may know.
Most situations call for you to use the Simplified Method to determine the amount of the return of basis per year.
Of course, if you had no basis in the pension or annuity, your retirement plan distribution is generally 100% taxable.
There are a lot of issues here - does this cover your question?
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
