To see what happens, simply "/summon poison", then right-click and "alter" it. Change the "regen" value to 1 and use it while your HP is full. The poison will ignore the max_hp, so you end up with a hp like "123/100".
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The alter action requires adminlevel 900. That admin level is intended to allow manipulation at a very low level in order to work around bugs. If you use it for fun of your own server, that is fine. But you are on your own.