e.g. it doesn't make sense to support billing in sub-currency unit amounts just by allowing it in your API definition, as you're going to need to batch that until you get a billable amount which is larger than the fee for issuing a bill. Even for something like $100,000.1234, the bank doesn't let you do a transfer for 0.34c.
For cases where sub-currency unit billing is a thing, it should be agreed what the minimal unit is (e.g. advertising has largely standardised on millicents)
