The concept of Soullight is similar to that of Karma in some RPG games.
Soullight represents your compliance with law and cooperation with the system.
Gaining and losing soullight
Soullight can be increased by completing faction missions (jobs). It is also increased by killing runners with a negative soullight.
Soullight is lost when you attack a neutral or friendly runner (except in a warzone) and is affected by the rules of the zone you are in. Apartment areas, for instance, inflict a very large soul light loss when killing even enemy runners, where killing hostile players in a dungeon zone has no negative effect on your soul light.
The amount of soul light lost when you kill a player with positive soul light depends not only on the rules of the zone type you are in (dungeon, city, wasteland, apartment, etc) but also the standing of your faction towards their faction. Killing runners of an enemy faction
Effects of Soul Light
Most importantly, highly negative soul-light will mark you as a criminal and make cop-bots and some other faction defenders open fire on you on sight, much in the same way guards of a hostile faction will open fire if you attempt to enter their HQ.
Also very important, characters with a positive soul-light have one "safe' locked slot on their quickbelt- slot #1. The item in this slot will never be dropped in your quickbelt upon death, making it a safe place to put your primary weapon if you don't want to risk dropping something like a Wisdom of Ceres weapon upon death.
In addition, with positive soul light, you will drop only a single item in your quickbelt. This means in warzones that reduce the number of items dropped by one, players with positive soul light will not drop anything at all. The more negative your soul light is, the more items you will drop.
Soullight also affects the security of your belt. Higher soullight provides a secure belt, while lower values make it easily hackable.