It's the raised portion on the tail end of the body that surrounds the switch. It basically makes the switch recessed so that the light can "tail stand."
The "shroudless" body has the switch standing proud from the back, making it easier to get to but does not allow the light to stand on its tail.
The Tricap on your new light is basically shrouded for tail standing but with three scallop-shaped cut-outs to allow easy access to the switch.