Vitality and Stamina

PC and monster abilities all cost hp, basically!

Some are on recharge or whatever, but the fundamentals are the same: casting a spell, breathing fire, or activating certain martial abilities aren't limited as a per-day effect but instead based on hp expenditure.

To even things out with healers these vitality points are actually a reduction of maximum hp. E.g, to cast a 5th-level spell you must reduce your hp maximum by 9 points.

Casters have an additional pool of temporary hp that they can use - these are their spell points. They can only use these temporary hp to cast spells, though some classes (abjurers, e.g.) may be able to convert those into actual temp hp or otherwise be able to turn it into healing and stuff.

If a spellcaster runs out of their spell point temporary hp they can dip into their 'real' hp, potentially hurting themselves very badly. If a caster hits 0 hp as a result of casting spells they fall unconscious rather than dying. The exception here is if they spend more hp than they have (so, for example, if a caster has 3 hp left and casts a 2nd-level spell they're at 0 and unconscious, but if they have 3 hp and spend 5 to cast a 3rd-level spell they are immediately dying and make their first Death saving throw at the beginning of their next turn).

How does this interact with healing?

If a PC casts a spell and then gets healing, what's the average 'exchange?'

Spell Level --- Healing --- Average ---- Cost --- Difference

       0                     0                  0                  0                0

        1                   1d8+2            6.5                 1                5.5

        2                  2d8+2            11                   3                8

        3                  3d8+3            16.5               5                11.5

        4                  4d8+3            21                  7                14

        5                   5d8+4           26.5              9                17.5

        6                  6d8+4             31                 11                20

        7                   7d8+5            36.5              13               23.5

        8                   8d8+5            41                 15               26

        9                   9d8+5           45.5              17               32.5

So we can see that a PC spending their own hp would normally be better off casting two or three spells, then getting healing 'for cheap,' except that the spellcasting reduces their maximum hp instead.

Spell points are recovered just like hp, at the end of a short rest or a long rest. You have to use the same pool of resources for both: if your astral self is exhausted your physical body can't recover, and if your physical body is recovering your astral self is stagnant.

This spellcasting operates because every individual has a reflection of themselves on the Astral plane that fuels their magic. This Astral self can be harmed just like the physical body, and certain creatures or spells can directly attack an individual's Astral self, reducing their ability to cast spells or channel energies.

Martial characters just get more hp, and temp hp, and can spend their regular hp to fuel their abilities.