Documenting EACA mandate exemption

We have a spinoff of a grandfathered PE from a grandfathered plan into a new spinoff plan.  I don't think MEP/PEP/SEP matters here but help me if that's not right.

That spinoff plan will be 001 for the EIN with a 1/1/2025 original effective date.  Assuming that’s the right original effective date for the spinoff plan (as compared to the original adoption date of the PE into the grandfathered plan…)

Where/how does the document distinguish this grandfathered 1/1/2025 plan from a new non-grandfathered 1/1/2025 plan?  In other words, where/how do we inform the participants/IRS/the world that this plan is NOT subject to the EACA mandate?

Depends on the plan document. All of them that I've seen (or remember) do have a spot for this. Usually either early on in the Adoption Agreement, or in an Appendix. In our documents, for example - the 401(k) doc it is on page one of Appendix A, whereas for the new Cycle 2 403(b), it is on page three of the Adoption Agreement. I'm sure you'll find it if you look through the AA and/or Appendices.