These are the plants that float on the top of the water, with its roots dangling underneath.
These plants can be pushed down in your substrate and will root in it. Stem plants are also very easy to take cuttings from.
These plants don´t want to be buried in the substrate. Bulbs should be placed on the substrate, so they can shoot their roots down, while rhizomes and epiphytes can be fastened to a rock or a piece of driftwood.