Please see this answer from KerriP.
No a garnishment is not the same as tax withheld. Tax withheld is in box 6 and is for current years Federal Income taxes with held for 2017 tax liability
A description of an amount in Box 3 includes a garnishment or Levy
A garnishment or levy is for overdue federal taxes, or other specific over due debts.
Per the Social Security Administration:Treasury benefit payment offset, garnishment, and/or tax levy – If we show an amount in this legend, either the Treasury Department applied that amount to a debt or debts owed to other Federal agencies; benefits were withheld to pay child support, alimony, or court ordered victim restitution; and/or benefits were withheld to pay debts to the IRS . Direct inquiries about what type of offset is involved to SSA at 1-800-772-1213. Direct questions related to federal taxes to the IRS at 1-800-829-7650.
Or see this website. SSA-1099 Garnishment.
The question is due to the fact that you did not receive the full amount of your SSA Benefits.
How can you offset your Income to reflex the difference?
The link you provided (=> SSA-1099 Garnishment) gives an error.
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