The Input/Output Reference Guide and his companion the Engineering [Engineering Reference Guide Guide](http://bigladdersoftware.com/epx/docs/8-5/engineering-reference are your friends.

If you take the Fan:ConstantVolume object for example, you see that the flow is actually a "Maximum Flow Rate". I think the name is pretty self explanatory: your fan just won't be able to go over this maximum flow rate.

The engineering reference guide has interesting sections, such as Air Loops - Determination of Air Mass Flow Rates