Pre-scaled Units refer to values expressed in the unit of the channel prior to the custom scale being applied. Usually, pre-scaled units are volts or amps since most channels will natively measure or generate signals of this kind. However, it is possible to associate a scale with a channel that contains a transducer explicitly supported by NI-DAQmx. In this case, the pre-scaled units are those used by the supported transducer channel.
For example, if you associate a scale with an analog input resistance channel, the pre-scaled units would be ohms and the scale would specify how to convert from ohms to the desired scaled units.
Scaled Units refer to values expressed in the final unit after NI-DAQmx applies the custom scale. You are able to define scaled units however you see fit for your application.
For example, if you assign a linear-position-to-voltage scale on a voltage output channel, the pre-scaled samples are in volts while the scaled samples could be specified in meters.