What records must an employer retain under this requirement--the participant's accrued benefit, or the contribution history?

The plan has to do what is reasonable. What is reasonable depends on the circumstances. You might also consider how the new DOL position on charging for QDRO work might fit into the scheme.

By the way, the "employer" does not do anything with plan records or data. The plan administrator or some other fiduciary has that function. More.

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