The perpetual hit point (or vitality reserve)

[malloyd]

Originally Posted by Mailanka

Like a point of DR plus a trait that counts as a hit point, but only for the purposes of calculating whether you're conscious or need to make survival checks or dead.

Actually I'd just count it as a hit point for all purposes plus a point of DR.

Suppose you have 10 HP, 1 point of this and take a hit for 20 points of damage. Your hit points start at 11, 20 points takes you to -9, and of course you didn't lose that one hit point, so you're at -8. That's where you'd be if you had DR 1 and 11 HP. Get hit again for 2 more points and you'd go to -10, but you don't lose that one point, so at -9. Again 1 DR, but no extra HP, you already "used" that. Seems simplest just to call it the combination.