This is what section 6.2.3. says:
6.2.3. Oxygen limit fraction (OLF)
If set to NITROX mode, in addition to tracking the diver's exposure to nitrogen, the instrument tracks the exposure to oxygen. These calculations are treated as entirely separate functions.The dive computer calculates separately for Central Nervous System oxygen toxicity (CNS)and Pulmonary Oxygen toxicity, the latter measured by the addition of Oxygen Toxicity Units (OTU). Both fractions are scaled so that the maximum tolerated exposure for each is expressed as 100%. The Oxygen Limit Fraction (OLF) displays only the value of the higher of the two calculations. The oxygen toxicity calculations are based on the factors listed in Section 9.3 Oxygen exposure.
And section 9.3 says:
9.3. Oxygen exposure
The oxygen exposure calculations are based on currently accepted exposure time limit tables and principles. In addition to this, the dive computer uses several methods to conservatively estimate the oxygen exposure. For example:
the displayed oxygen exposure calculations are raised to the next higher percentage value
for recreational scuba diving, the recommended upper limit of 1.4 bar PO2 is used as a default
the CNS % limits up to 1.6 bar are based on 1991 NOAA Diving Manual limits
the OTU monitoring is based on the long-term daily tolerance level and the recovery rate is reduced
Oxygen related information displayed by the dive computer is also designed to ensure that all warnings and displays occur at the appropriate phases of a dive. For example, the following information will be shown before and during a dive when the computer is set in NITROX mode:
the selected O2% on the alternative display
OLF% alternative display for either CNS% or OTU% (whichever is larger)
audible alarms are given and the OLF value starts to blink when the 80% and 100% limits are exceeded
audible alarms are given and the actual PO2 value blinks when it exceeds the preset limit
in dive planning, the maximum depth according to the O2% and maximum PO2 selected
Please note there are three things that can go wrong when using Nitrox:
1 - exceeding your MOD (this is the one you know about and it is seriously dangerous)
2 - exceeding the maximum time over which you can breath high PO2. and
3 - exceeding the maximum time over which you can breath moderate PO2.
Numbers 1 and 2 both lead to CNS toxicity with probable convulsions and thus drowning.
Number 3 leads to pulmonary toxicity, and is uncomfortable but not fatal.
Numbers 2 and 3 are measured against an allowable accumulation (CNS clock or Oxygen Tolerance Units), which Suunto calls 100% and gives you a warning at 80%. Section 6.2.3 says the OLF shows whichever of Numbers 2 and 3 is currently closest to your allowable maximum. As JohnN said, it is hard to get to that maximum doing recreational diving with recreational Nitrox percentages, but it is not impossible. Note, however, Number 2 leads to CNS toxicity, so pay attention to your OLF just in case.
