Now the rebirth one is a tricky one to explain as it changes per voc and spell.
So the easiest way to explain this is, As of 05 September 2019 (This is subject to change at anytime and if not updated may be wrong)
Each rebirth will give approximately 15 levels of HP and Mana.
If you get 5 HP per level you would receive + 75 HP per rebirth
You also receive More capacity that is based off the main potion weight for your vocation.
This capacity amount is equal to 50 potions per level
You also receive a damage boost per rebirth also, This depends on which spell you are using as each spell has a different minimum and maximum damage.
Lets run a 15 rebirth trial maths trial. a knight that gains 23 HP.
Would receive a total of 15x23= 345 HP per level, now rebirth 15X and you would have a total of 5K hp more.
You would be able to carry 750 more potions.
And more then likely be able to do more then 15% damage to a target with your spells.
However for 15 rebirths you would receive 5% less exp per rebirth, But at 15x rebirths you could eat a fish to receive the same amount of exp as normal making it eaiser.