New participants post stock deal - use plan year comp or prorated comp for QACA SH match?

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- Company A acquired Company B as a new wholly owned subsidiary via stock purchase. 

- Company B will terminate its 401(k) plan prior to close.

- Company B employees will participate in Company A's SH plan 10/1/2021 but will remain on Company B's payroll through 12/31/2021.

- Company A plan is a QACA safe harbor plan and provides that Company A will make matching and non-elective contributions based on pay for the plan year.

If Company B is not a participating employer in Company A's plan, does Company A need to use former Company B employee's plan year compensation when calculating non-elective contributions or can it use compensation from 10/1/2021-12/31/2021?

If Company B becomes a participating employer in Company A's plan, does Company A need to use former Company B employee's plan year compensation when calculating non-elective contributions or can it use compensation from 10/1/2021-12/31/2021?