Employee vs. Employer Contributions
The employee’s own salary deferrals are fully vested—meaning they are not subject to forfeiture and will be divided as marital property if earned during the marriage. However, most 401(k) plans also include employer contributions. Those contributions are subject to a vesting schedule. That means some portion may not belong to the employee if certain requirements (like years of service) haven’t been met.
The bottom line? If dividing this plan through a QDRO, document which contributions are vested and which aren’t. Only vested employer contributions can be awarded in the QDRO.

