It means that the pension administrator has to report certain items separately for box 3 out of what is reported in box 2a. You can read about it here.
The link is pointing to the 1099-R instructions. These instructions are not for you, but are instructions for the entity who creates the 1099-R that is sent to you.
Note that despite what is in box 3, all of the taxable distribution (box 2a) is included in taxable income and taxed as ordinary income.
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