Mouse hover on First button. When mouse reaches to border line of the button the mouse pointer changes to hand icon. Where as it is not same with second button. Move your mouse to second button border. The mouse pointer changes over to hand icon before it touches the border line of the button. Move around the button...you can see upto 10 to 12 pixel area from all four sides the button is active.
The Hit frame of the second button having a graphic larger than the original button. No matter what colors you place in Hit frame. It can't be seen while playing movie. Transfarent buttons are very useful when we need to define certain graphic areas as clickable links.
The following transparent text button is with 100 pixel width and 50 pixel height. You only see text but not its active clickable area.
The timeline screenshot for above button:
On stage the color of transparent (irrespective of color given to button in Hit frame) button :