How does MFP classify slow, moderate, vigorous

The Compendium of Physical Activities, which MFP uses as a basis for their pre-loaded activities, provides ranges of energy output in Watt ranges.

https://sites.google.com/site/compendiumofphysicalactivities/Activity-Categories/conditioning-exercise

If you are you using the pre-loaded MFP activities for "Stationary bike," you may have to choose which activity comes closest to your perceived intensity if the equipment does not provide a readout of the energy expended. If you have to guess, it is best to choose one, log that for a few weeks, and adjust your eat back percentage of Calories earned based on actual weight change results.