Employee vs. Employer Contributions
401(k) accounts often contain two types of money: what the employee contributes and what the employer may match. Contributions made by the employee are typically fully vested. But employer contributions may be subject to a vesting schedule. If the marriage ends before full vesting, the alternate payee may not be entitled to the full employer match.
Ask for the plan’s Summary Plan Description (SPD) and a vesting breakdown when you’re starting the QDRO process.

