Trip mileage reimbursement — distance driven × your rate per km/mile.
Use whichever distance unit (km or miles) matches your rate.
No — it just multiplies distance by rate, so as long as the rate you enter is quoted per the same unit as the distance (both in km, or both in miles), the result is correct either way.
It simply doubles the one-way reimbursement, assuming the return leg covers the same distance at the same rate — useful for quickly totalling a there-and-back trip without re-entering numbers.
It scales your entered rate up to show what 10 units of distance (10 km or 10 miles, matching whatever unit you used) would cost — handy for sanity-checking a rate or comparing it against a policy that's quoted per 10 km/miles.