Employee vs. Employer Contributions
Most employees own 100% of their salary deferrals. However, employer contributions may be subject to a vesting schedule. In divorce, you can only divide what was actually owned (vested) by the participant on the date assigned in your divorce judgment. If the order mistakenly attempts to award unvested amounts, the plan administrator will reject it.
A good QDRO will specify how to handle both vested balances and contributions that vest after the valuation date—some couples choose to include post-divorce vesting in the division (if the plan allows it), while others do not.

