Power of Existence is the basic essence in the Shakugan no Shana universe. It comprises practically every living and non-living thing in the world, and is also needed by Crimson Denizens and Flame Hazes.
It is usually undetectable by humans unassisted and only Crimson Denizens and Flame Haze are the only ones who can usually see Power of Existence, which usually comes in the form of a blue flame (anime only).
Power of Existence is something that makes others remember us. It can be found on any object, be it living or non-living; it is also somewhat present on dead people or animals.
Every humans are born with a certain amount of Power of Existence. This amount increases or decreases depending on how influential they become. Persons who pass away or is killed by means other than being consumed by Crimson Denizens retain a small amount of Power of Existence, which allows other people to remember them even though they are no longer living.
Usually, a person who lives in a high-class society like royalty or the elite acquires a larger amount of Power of Existence because of the amount of influence of their respective families.
Status is usually part of the criteria set by Crimson Lords who seek their partners: Margery Daw is the daughter of an aristocrat, Wilhelmina is presumably a part of the royalty, Khamsin is a Middle-Eastern prince, Karl Berwald is a nobleman and Sale Habichtsburg is an aristocrat's famous entertainer.
Torches are people who lost their existence because a certain Denizen has eaten it, in other word, they're all dead. The suddenness of their disappearance can distort the world's balance. To prevent that, Flame Haze created a "thing" instead of the human by put Power of Existence inside of them. When the "flame" inside it flickers out, he/she disappears, as if he/she did not exist in the first place.
Flame Haze, undergo a different process; when they agree upon the contract between a Crimson Lord, the entirety of the Power of Existence that he/she possesses will be taken up Crimson Lord. The Crimson Lord then will replace it with their own Existence. This make the person loses his/her identity, no one will recognize him/her anymore.