What is the difference between Fill & Opacity in layer

What is the difference between Fill and Opacity? Here are my layers, I had this layer selected, I lower the opacity. Everything goes down, the Layer plus the Drop Shadow, but Fill will lower the Layer, but not the Layer Style. That is the quick version of the difference. And in my life I never use Fill, sorry Fill. 

The long version is, we're going to add some text. Move it around. And what you'll find is, Opacity, when we slide it down, makes it transparent, and Fill does the exact same thing. And I'm like, "Why do I have both?" It's just so that if you add something, like a Layer Style, let's say a Drop Shadow, add a Drop Shadow, now if I lower the Opacity, it does both the Layer and the Drop Shadow, whereas Fill does the Layer but not the Layer Style. There you go.

 
That is the difference, and if you're like me, this could be the first time you've ever lowered the Fill and not the Opacity. So for me, 99.9% of the time it would be the exact same thing, and I don't need Fill, sorry Fill. Thank you pug in sweater; thank you, Charles. Pugs are awesome. All right, next video.