These variants correspond with a few of your subtle bodies;
Etheric Body
Astral Body
Mental Body/Concrete Mind

Those range in level of density as well as difficulty; Etheric being the hardest to achieve and Mental generally being the easiest.

From Hardest and Most Dangerous, to the Easiest and Generally the Safest:

Etheric Projection
Called or referred to as a ''near death experience''.
Affects May Include~ All vital signs pointing to death: your heart rate speeds up and eventually stops, no respiration/breathing, severe temperature drop and so...
One may achieve full or highest potential through this variant, also ascending into truest form a.k.a higher self (HS).
The soul and mind as it is traveling will feel anything and everything that happens to the physical body. If someone steps on your fingers while you're Etheric Projection, you're going to have a bit of pain on your hands ;). If someone stabs your body multiple times violently, I'm afraid you won't be coming back and instead will stay there on the astral.
In This Phase/Variation~ One is practically a ghost/phantom. If seen, one will be viewed as such.
Possible Skills~ Think of what an apparition or your true form can do and apply it to the world around you (normally this plane). Moving to any plane; below, on or above this one.

Astral Projection
The Most Common Variant (happens sometimes when one isn't even aware)
Some dreams or forcing oneself to dream.
Affects May Include~ Seeing one's own body during ''take off''. Intruding others' dreams. Meeting others on the planes. One's life will be COMPLETELY vulnerable due to not being able to feel the physical body.
In This Phase/Variant~ Premonitions, third eye opening and other things dealing with divination might happen or strengthen. Mos thing that happen on the planes are done in astral form.
Possible Skills~ Divination occurrences, a tad bit more than the Etheric Body. Traveling anywhere above the physical plane.

Mental Projection
Throwing one's thoughts/mind to another plane, almost like lucid day-dreaming.
Affects May Include~ Loss of memory fro physical body as well as more basic and simplistic forms of divination but that is for another day.
In This Phase~ Projecting while still in live casing or physical body.
Possible Skills~ Basic divination occurrences. Traveling not too far above the physical plane.