According to Wiki a fixed-point number representation is a real data type for a number that has a fixed number of digits after (and sometimes also before) the radix point. (More info to be found here)
Within SAP (BW) fixed point arithmetic is sometimes being used by function modules

When we’d made a copy of standard SAP function module CONVERT_TO_LOCAL_CURRENCY to Z_CONVERT_TO_LOCAL_CURRENCY

the result/outcome of both function modules, when testing them via SE37 and using the same test set (ceteris paribus), was no longer equal.

Within the original function module 100 EUR was converted to 80 USD (for example), while the Z-copy converted 100 EUR to 80000 USD (A factor 1000 to big). After some extensive debugging we found out that the above anomaly was being caused by the “fixed point arithmetic” checkbox of the function group (to which the function module belongs).

After the above mentioned copy of the function module (and its corresponding group), the “fixed point arithmetic” checkbox of the function group had been checked automatically (by the copy action).

After de-checking this “fixed point arithmetic” checkbox, both function modules (the original and its z-copy) produce the same output again!