Employee vs. Employer Contributions
Most people are surprised to learn that not all of the 401(k) funds might be available to split. Employee contributions (the amounts the participant put in from their paycheck) are always 100% vested. Employer contributions (the company match) may be subject to a vesting schedule.
If only a portion of the employer contributions are vested at the time of divorce, only that vested amount can be divided in a QDRO. Unvested amounts go back to the plan if the participant leaves the job before becoming fully vested.

