Graphics from the S ROM are used for stationary life bars, credit counters, on screen text and other graphics that don't need to move around. In the reference image, S ROM graphics are used for:
"SELECT ENEMY"
"TIME" and "5"
"COM"
"LEVEL-4" and "CREDIT 00"
They can't scroll or scale and they always appear "on top" of all sprites. This can cause S ROM issues to completely cover the screen while the opposite isn't true.
S ROM D0 (FIXD0) stuck high which creates jailbars covering the whole screen
Data outputs from the S ROM go through FIXD0~FIXD7 on the cart slot. If any one of these traces is bad then it can cause glitches that cover the entire screen. A single bad output can cause every odd (or even) column of pixels on screen to be obscured. Sprites below can appear fine with half the screen covered up.
Example: