SI screens are between the focus screen and prism.
They have semi-silvered areas that light up from LEDs in the prism housing
to indicate the area being focused by the AF system.

The SI screens on the 5D and 7D are completely different and are not interchangeable.
Keying projections cast into the screen set into corresponding recesses in the mirror box.
In addition, there is a lifting tab on the front of both screens.
Rulings and other viewfinder marks are on the SI screens as all focus screens contain only a fine Fresnel and micro matte surface.