Broken Enlarger Glass?

I have an Durst M600 Enlager and the part of the head sildes up (it’s kinda of a square box) to put the negative tray in fell off and the glass inside chipped. I have to get it replaced but I don’t know what the glass is called that is chipped, or what the glass does or is for. Sorry its kind of hard to explain. Any help would be great.