Employee vs. Employer Contributions
In most 401(k) plans, the participant contributes a percentage of each paycheck, and the employer may offer matching funds. These employer contributions often have a vesting schedule, which can dramatically affect how much of the account is divisible.
If, for example, the participant is not fully vested in the employer match, only the vested portion can be divided in the QDRO. Make sure the QDRO clearly defines whether the alternate payee gets a share of just the vested balance or a proportional share that includes future vesting rights. This should be negotiated during the divorce and clearly written into the order.

