The .045 or so would be used as the outer layer on the back instead of glass, the thinner .018 or so is sometimes used under like .030 glass usually on the belly more so for looks, not sure how much performance you get using it on the belly but Bowyers who use it that way could tell you more, I've used it for the back only with a glass belly and can tell the performance is better on my designs than a glass back and belly, but then I am still exploring different lay ups, it is much more expensive than glass and the performance depending on design may very.