Spin-off ADP/ACP question

An employer sponsors a 401(k) plan for their employees, plan year is 1/1 to 12/31. We'll call this Plan A. They spun-off a certain division of employees into a separate (new) plan on 9/1/2023. We'll call this Plan B. These individuals did NOT incur a termination of employment. 

When testing Plan A for 1/1/2023 to 12/31/2023, the spin-off individuals will be included, but it will only include their contributions from 1/1 to 8/31, correct? As for the compensation used on the test for Plan A (for the spin-off group), I think it is going to be dependent on the provisions of Plan A, and how it defines compensation for the ADP/ACP test. Would that be correct as well?

Depending on the plan's provisions - the compensation used for the spin-off group on the Plan A test could be from 1/1/2023 to 12/31/2023, or it could be from 1/1/2023 to 8/31/2023, if the plan excludes compensation while the participant is not an eligible employee (for Plan A). Thoughts?

What does the plan document say? 

I find the most aren't specific about testing compensation - which is different from compensation for deferral or accrual purposes (such as compensation from entry). 

Separate plans does not necessarily mean separate testing. Sounds like it is a single employer with two plans. Likely everyone has to be included in both sets of tests, unless you can show with some non-benefiting testing that the coverage tests passes for each group. That testing option might not be available until next year, the year after the spin-off. 

Typically the rules require (and the document language mirrors) that regular compensation ( not self-employment earnings) across a single employer has to be considered for the plan testing. So if a W-2 employee has compensation from two divisions, but the same employer, it doesn't usually matter which division it was from. If this is two large plans there might be special rules. 

Read the document's section on plan compensation to start. it should shed some light. Not just the adoption agreement, the basic plan document if there is one.