Employee vs. Employer Contributions
Participant contributions (also known as employee contributions) to a 401(k) are always 100% vested. Employer contributions, however, may be subject to a vesting schedule. This means only a portion of the employer-funded balance may be available to divide, depending on how long the participant was employed by the company prior to divorce.
An effective QDRO needs to address these details explicitly—clarifying whether the alternate payee is entitled only to the vested portion or if unvested amounts are to be considered in any way in the division.

