What's the purpose...

What’s the purpose of the screw? Stabilization? An issue?

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The screw is for breakers that back feed the bus. If there were no screw, and the breakers popped out, you’d have live breakers and wires flopping around.

I have seen other mechanical devices used for this purpose as well.

You can’t just go drilling holes in any breaker though, not that I’ve ever seen that.

Here’s the NEC requirement:

Cool. Thanks.