Essence magic, a caster attunes their whole aura to their environment (empathy) and draws in essemce power from a standing field to cast spells. hence no armor. Effects tend to be very powerful, but direct and unsubtle.
Psionics are powered by mental power of your self, which likely means food in some ultimate progression of source. (i.e. you use it and it makes you tired and hungry, like climbing a ladder or walking.). The effects tend to be far weaker, since you self power them (in terms of actual psions compared to spells.)
Mentalism casters use their force of will to draw essece power from a standing field via their aura, but specifically via their head. (hence no helmets). They have less powerfull but more focused and subtle effects. (in terms of spell lists).
Hybrids must abide by the restrictions of both realms, so no armor or helmet. . .the me/es hybrid is full body aura linking and also drawing via their will/mind for their hybrid base lists, then combining, weaving or mixing the power to cast the spells.
essentially, where with a psion, the user is doing ALL the work, with a spell the user is working to use and shape an ambiant force that does the work. . .Magic in this instance is sort of like using a lever to do something, less direct effort, more end result than trying to do the same job by hand without the lever.
