Digital BW, The Print

I use layers a lot with various modes such as "soft light" 
or "overlay."  It appears that the opacity and fill sliders have 
different effects and there are times I use both in adjusting a 
layer.  The function of the opacity control is obvious, but I have no 
idea what "fill" refers to.  Am I imagining differences in effects 
between the two?  Is there a standard workflow for dealing with 
the "intensity" of layers?  I sometimes adjust opacity first and then 
make minor adjustments with the fill slider and sometimes do it the 
other way around.

Any help appreciated.

Steve