1. Employee and Employer Contributions
The first thing to know is that 401(k) plans consist of various contributions, including employee deferrals and possibly employer matching or profit-sharing contributions. In most divorces, the division focuses on contributions made during the marriage—or more specifically, from the date of marriage to the date of separation or divorce.
Employees are always 100% vested in their own salary deferrals, but employer contributions may still be on a vesting schedule. If the participant is not fully vested in those amounts, only the vested portion can be divided under the QDRO. Any unvested portion will remain with the participant or may eventually be forfeited if the employee leaves the company.

