‘Have you made nondeductible, after-tax contributions to the plan? ’
If you have made contributions to your 401(k) that weren't deducted from your annual income in the year of the contribution, you need to check with your plan administrator to ensure that these contributions are separated from the remaining retirement funds and not commingled with other IRA assets. Failure to do so would require you to file an additional IRS form (8606) with your annual tax return to protect what should be tax-free withdrawals of those contributions.
The fifth question is here.