Small levels within a larger level or category.
Sublevels help to organize things into smaller groups.
Examples:
In the game, there are different sublevels you have to beat.
The book is divided into sublevels to make it easier to read.
She is studying the sublevels of the ocean.
The sublevels of the cake are each decorated differently.
The sublevels of the building are like little worlds inside the big world.
The sublevels of the garden have different kinds of flowers in each one.