Employee Contributions vs. Employer Contributions
Employee contributions, including salary deferrals, are always the property of the participant (and thus can be divided). However, employer contributions are often subject to a vesting schedule. If the participant is not 100% vested, a portion of those employer contributions could be forfeited upon job termination.
Your QDRO should clearly address how both vested and unvested portions are to be handled. For example, it may only divide the vested portion or specify reallocation if unvested funds become vested later on.

