Maximum means, well, maximum. The card is not specified to do more.
Outside that range its behavior will be either a refusal to function
(error message) or unpredictable behavior.*
*In at least one case that I am aware of- the PCI 6110- setting a
sampling rate above the maximum specified leads to sampling at half of
the rate set. I have a feeling NI didn't really want this to happen.
That's what I mean by "unpredictable."