What you must do
Where one of the sixteen enumerated sections conditions allowability on prior written approval, obtain that approval from the Federal agency (or, for indirect costs, the cognizant agency for indirect costs) before incurring the cost; for other special or unusual costs, seeking advance written approval is the mechanism to avoid later disallowance or dispute.
When it applies
Budget includes any cost category on the § 200.407(a)-(p) list: cost sharing (200.306), program income (200.307), budget/program plan revisions (200.308), fixed amount subawards (200.333), compensation—personal services para (h) (200.430), compensation—fringe benefits (200.431), equipment and other capital expenditures (200.439), exchange rates (200.440), fines/penalties/damages/settlements (200.441), fund raising and investment management (200.442), goods or services for personal use (200.445), insurance and indemnification (200.447), organization costs (200.455), pre-award costs (200.458), rearrangement and reconversion (200.462), travel costs (200.475)