Make-up contributions are employee contributions that could have been deducted from your pay earlier, but were actually deducted later because of an error.
When you are erroneously put in CSRS, CSRS Offset, or Social Security-Only rather than FERS, you are allowed to make up the TSP contributions that you could have made had you been in the correct retirement plan.
By law, your TSP make-up contributions must be made as payroll deductions. You can't pay your TSP make-up contributions by check or rollover. Subject to the provisions of the TSP error correction regulations, you can decide how much you pay in TSP make-up contributions and how long you want to take to make the payments. TSP make-up contributions are treated as tax-deferred compensation for the year in which they are made up, but are subject to the elective deferral limit(s) for the year(s) in which they could have been made. So, your make-up contributions will reduce your taxable income for the year that you actually make the contribution.
If you are in FERS and decide to pay TSP make-up contributions, your agency must also pay any attributable Agency Matching Contributions.