Id is the texture I belive, haven't tested...
Fade is... fade? It talks for itslef hopelyfully...
X1,Y1,X2,Y2 is the points from a side,top,another side, bottom. Use theese four points to draw the box.
r,b,g,a is Red, Blue, Green, Alpha
Alpha is how visible the box is overall, so zero is an invisible box and one is a totally visible one that blocks every other sights. Now ofcourse you can have 0.3 or 0.7 or any number between 0 and 1(this rule also comes to r,b,g!)
EDIT: Note that it's impossible to draw it in any angle, it's stay vertical/horizontal what so ever!