For a multi-stage system, which stage is for what fan speed?
Not sure you are giving enough information here to know where to begin to help answer. But I'm going to make some assumptions as I think I know where you are going.

Are we talking about a forced air furnace? I'm assuming so. 

Are you meaning to ask "with a variable speed" or "2 speed" or "multi speed" blower motor what is the trigger for the speeds to kick in? Assuming so:

This is going to depend on furnace manufacturer, thermostat model, and the system installer. For example, furnace Control boards and smart/programmable thermostats are programmed to react *(in some cases as a smart-learning mode) to what speeds affect the temperature stability in the home and will self regulate. Large swings in temperature can trigger high speed regardless of program.

Some systems also allow you to set it static, either high, or low. Also, some installations have external temp switches and will only use high speed when the delta temp outside is below or above a set point.

So there are many things that can control fan speed. When it comes to multistage, this can be referring to dual heating units, or different set points for BTU generation.