I know we all probably use the opacity slider on the Photoshop layer, but how many of us have ever use or know how/when to use the fill slider? Well, famed retoucher
Natalia Taffarel explains in her Model Mayhem tutorial on the difference between the two.
This is the original text with glow created from layer FX with opacity and fill at 100%
When you decrease the opacity, both the white text and the glow disappear
When you decrease the fill, only the white text disappears. The basic difference between opacity and fill is that the fill only affects the content (white text), but not the effect (the glow)