"What IRS Publication is listed for the reimbursable business expenses that IRS allows?"

There is no One publication. There are lots of things that would be examined. It matters what entity type this is and the relationship of that person to that entity. It matters if this is travel, lodging, meals, supplies, health premiums, health care, tuition, child care, HSA, FSA. You might see per diem is often used instead of Accountable Plan requirements, because it specifically is provided as a paperwork reduction. But there are multiple per diem, including truckers that are subject to DOT per diem. There are location-specific per diem.

You can google this:

irs accountable plan

And you can read:

https://www.investopedia.com/terms/a/accountableplan.asp

https://www.investopedia.com/terms/n/non-accountable-plan.asp