Rebirth becomes available after reaching level 400 at SL10. This resets your stats and stars to 0. Why would you do this? Because each Rebirth Star grants:
100% buff to PMC, Training Gains, and EXP
5 Rebirth Skill Points
You restart at SL0, Level 1, but with a permanent 100% boost. Security Level Stars stack with Rebirth Stars, so you could have 10 SL stars plus Rebirth stars for a massive 300% boost.
You can rebirth repeatedly, but each time requires 50 more levels:
First rebirth at level 400
Second rebirth at level 450
Third rebirth at level 500
And so on. It gets progressively more difficult.
Rebirth Skill Points can be spent on powerful upgrades:
Hitman: Reduces attack energy cost by 5% per point
Boss Grinder: Reduces boss fight cooldown by 2 minutes per point
Ultimate Trainer: Increases training gains by 3% per point
Ultimate Crimer: Increases EXP from crimes by 3% per point
War Master: In wars, attacked players contribute 2x base points (with 1 point) plus +1x per additional point