Any structural elements that constructed below the ground floor level is substructure and all structural elements above ground floor slab is a super structure

In construction, the difference between a sub-structure and a super-structure has something to do with which part the construction is done, based on ground level. Any structures that are done below the ground level is sub-structure. That could range from a simple foundation to a basement floor, or more complex structures such as subways or underground tunnels.
Super-structures are those portions of a building or any structures that are visible (from the ground level up). It could be a simple residential house to a multi-story building or a complex structure.

I think a substructure is a module that is built as a unit then incorporated into the building – perhaps a corner with diagonal bracing to which straight beams are attached in various directions.
A superstructure is a self contained module that is added to the main structure, usually on top (superior) A dome, a cupola, an observation deck, all might be added as superstructure. On a ship, the superstructure is the part that protrudes above the hull and deck of the ship – most obviously the multi-story stuff along side an aircraft carrier's deck or at the back of a large oil tanker.