A FixedDecimal is similar to a ScaledDecimal, but different in certain select ways. It's primary purpose was to be able to represent precise decimals for such things as representing money - where ScaledDecimals leave something to be desired. For instance, with ScaledDecimals, you get:
(33.333s withScale:2) + (33.333s withScale:2) print it yields 66.67s
but with FixedDecimals, you would get:
(33.333 asFixedDecimal: 2) + (33.333 asFixedDecimal: 2) print it yields 66.66.
So, FixedDecimals round the numbers to the exact scale you specify - converting a float (or fraction) to a FixedDecimal and back will not necessarily return the starting number, unlike ScaledDecimals.