which are allowable expenditure ?

Answer 1

IRS.org

has the IRS definitions and examples of allowable expenses for income tax deduction purposes.

Businesses also have separate allowable expenses.
Corporations have separate allowable expenses.

Also, some states have allowable expenses that are different from IRS. For example, the State of Pennsylvania does not allow depreciation from the state tax form, but the IRS has this as an allowable expenditure.

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