From International Building Code (IBC) Table 1604.3Deflection Limits :
Note i) reads " l = Length of the member between supports. For cantilever members, l shall be taken as twice the length of the cantilever. "
Therefore for each "position / condition" in the table, the allowable deflection is some fraction of length of the given member. For example: Roof member supporting plaster or stucco ceiling (member supported at both ends) :: L or Lr = (length of member)/360.
I'm going to use imperial units. Let's say you have a 20' span steel beam that's supporting a floor. The deflection under live load, L, would be
20ft x 12in/ft / 360 = 0.67in
The computed deflection of your steel beam must be less than 0.67in. So let's say you use the 384wL^5/5EI formula (if I remember correctly...probably not) for pin-pin deflection under uniform live load, and you get a deflection of 0.80in. That's more than 0.67in and it's not allowed, so you need to upsize your beam with a higher moment of inertia.
Remember to read all the little notes a through i. They don't usually apply, but sometimes they do! Hope this is helpful and what you were looking for, because I'm not 100% sure what you're actually asking. Good luck on your deflection endeavors.
