This is a document-specific setting, so you can have documents that show invisibles, and documents that don't. You can change the default setting in preferences. You will have to be at the manager level in Mac Control to do this. Also, no programs can be running while you do this. The reason this adds speed is that Claris loads all of the fonts while it is starting up, so it will work a bit faster if you slim down your seldom used fonts.
Libraries are versatile.
I tried to put sound in one. I got it there, but it wouldn't play. Neither would the movie. It works real good for graphics and text though. It is a quick way to change pages.