Well, FAST dicharges the battery then charges and measures how much charge put in the battery, it is mostly useless especially for older batteries that convert much of the charge to heat. NOR charges, then discharges and measures how much got out of the battery, so it is more accurate as capacity measurement but not exactly spot on because it doesn't let the battery cool off first and dissipate any surface charge; still good for comparisons however. Discharge is performed at 250mA when the charge current is set to 300/500mAh and at 500mA discharge when charge is set to 700/1000mA.
The mR measurement is not spot on but can give a rough idea if the battery develops high internal resistance. Most normal NiMH register exactly 45mR which make me believe this is a sort of fixed value when the charger detects a NiMH, eneloops on the other hand most of the time are 31-42mR which for me shows they are sufficiently better constructed than most NiMH around to the point this difference to be detectable by the charger. Li-ion start at about 20-30mR and as they age this goes up to 100+mR, I have two such Li-ions which can't go below 60mR but they are heavily used and old so no surprise here. This measurement is mostly affected by the cell temperature, you get the most accurate reading when the battery is cold.