Smallest order value in the QUOTE currency — for BTC_USDT, a minimum in USDT.
This is the EFFECTIVE floor: the larger of the configured minimum (the pair's own, or the
quote currency's when the pair sets none) and the step 10^-digitsAmount. An order value is
rounded down to digitsAmount decimals, so anything below that step becomes zero and is
rejected whatever the configured minimum says — on a pair with digitsAmount = 0 the real
floor is 1 quote unit even when the configured minimum reads 0.5.
Never null: a floor always exists.
One rule cannot be expressed here: what you receive net must exceed the taker fee, or the
order is rejected as dust. That depends on your own fee rate, so it is not a static property of
the pair. In practice it only bites right at the boundary, which this floor already covers.