"NO" or normal operating limits are below "NE" or never exceed limits, which are the same as "MO" or maximum operating - the difference here is terminology.
What determines the maximum operating speed?
Firstly structural considerations are used to determine a theoretical Vd/Md - the design limit. This is the limit to which flight testing is allowed to take the a/c.
If (as often happens) a handling or flutter deficiency is found below that, then a lower limit of "Vdf", or flight design limit is laid down.
Finally , Vne/Mne/Vmo/Mmo is determined as a margin below Vdf. The actual margin is a function of the certification standard - for light aircraft codes it's simply 10%, for "higher" codes like part 25 it's rather more complex.
The use of Vno/Mno is about 2-tier limits. Basically you are given a limit which you should not NORMALLY go beyond, but if you do you know that up to Vne/Mne there is no risk and you need not report it. If you go beyond that there is a risk of structural damage and/or unacceptable handling characteristics being met. Some organisations prefer 2-tier limits, some do not.
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