Employee vs. Employer Contributions
Only the portion of the plan acquired during the marriage is considered community or marital property. Contributions made before or after the marriage are usually not included unless your divorce settlement says otherwise.
Many 401(k) plans also include employer contributions that are subject to vesting. The portion that is unvested at the time of the divorce may not be available for division. It’s important to obtain a vested balance as of the division date to determine how much the alternate payee (former spouse) is entitled to.

