Inverted monotube means the "body" of the shock that contains the oil and the gas chamber is attached up-side-down to the body. So rather than the piston sticking out the top and going through the top mount, the bottom of the shock is attached to the top mount and the piston goes down to where the mount bolts to the hub. This has a few advantages.
1. lowers the "sprung mass". because the shock never moves, only the piston move. The ('heavy") shock body is attached to the car. Lower mass = better control. like lighter wheels help suspension to work better etc.
2. Creates a more rigid set with a "tube inside a tube" for the front suspension which helps keep geometry stable under high corner loads.
Best way to identify this is the "shiny" part of the shock that is visible usually the piston shaft would be ~3/4" diameter, maybe a bit more, with an inverted monotube shock the part you see is the shock bod which could be 2" diameter.
Left pair: Inverted monotube, Right Pair, Monotube.
(not for sentra, but example of how it looks)