Massive Damage and Non-Lethal Damage use hybrid rules from D&D and d20 Modern.
Your Massive Damage equals 50 HPs or your constitution score multiplied by your level, whichever is higher.
Some forms of damage are so superficial that they can knock you on your ass, but they don’t have any permanent effects. This is non-lethal damage, or “NL” (as opposed to lethal damage; “HP”). In Phoenix, unarmed strikes made by people without the proper training (i.e., the Combat Martial Arts feat) or without super-strength cause NL. Some weapons and powers are specifically designed to do only NL. Some other things, like heat or exhaustion, can cause NL as well. Specific things that cause NL will say so in their descriptions.
Tracking Non-Lethal Damage
Keep track of NL damage as a running total; do not reduce it from your HPs. When your NL equals your HPs, then you become staggered. When your NL is greater than your HPs, then you become unconscious.
For example , you have 50 HPs in total. You take 20 HPs lethal and 20 NL. If you were to take 10 HPs damage (i.e., reducing you to 20) or 10 NL damage (increasing you to 30), you would be staggered. If you took were to take 21 points of either lethal or non-lethal damage, you would fall unconscious. For the sake of simplicity, you can list your damage as “HP/NL.” In the example above, you would have “30/20” (30 HPs remaining and 20 NL damage).
Making Non-Lethal Attacks
You can choose to do NL with a mêlée weapon that normally deals HPs, including unarmed strikes, but you take a -4 penalty to attack. This means that you attack with the “flat of the blade” or otherwise pull your punches. The feat Merciful Mauler eliminates this penalty. You can also do lethal damage with non-lethal weapons, including unarmed strikes, but you again take a -4 penalty to attack.
Non-Lethal Massive Damage
If you fail a massive-damage roll as a result of taking NL, then you merely fall unconscious. You do not die.