What is the purpose of this?

[hoosier inspector]

In the basement underneath a gas fireplace, I was writing up the cut joist and at loss to describe the metal structure, why is plywood covering it, and is that okay? Is this to attach another appliance?

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[AHI in AR]

That's a firestop and a section of metal chimney in it. It would appear that they installed a second chimney system to allow for a future installation of another fireplace in the basement. The plywood is to stop drafts, I assume.