A phenomenon unique to the Celestial Exalted, and a mechanism to act as population control thereof, a Spark of Exaltation (or simply Exaltation, or insert-celestial-type-here Essence) is a shard of a God's power, modified to harmonize with a human body and soul. The Exaltation acts as the power source for the Celestial Exalted, allowing them to surpass human limits of all types. In Yu-shan's infinite departments and bureaus, Lytek, the god of Exaltation, is supposedly in charge of receiving the Exaltations of those who have died, cleaning off the detritus of memories of that life, and arranging for a new host.
In reality, traces of memory cling to Exaltations beyond what Lytek can accomplish, and some Celestial Exalts find themselves able to coax these back into full detail. Also, the Sparks seem to have at least a minimal guiding from their patron gods, depending upon the accounts of Exaltation, and the type thereof in specific. Luna seems to take an active role in all Exaltations under her, personally appearing for a variable length of time to the new Lunar Exalted immediately at their own Exaltation. The Unconquered Sun, on the other hand, speaks to the Zenith Caste among his chosen, but rarely to others. The Five Maidens, further down the spectrum, usually do not appear to their chosen at all, though the Sparks of Exaltation that they bestow are integrated into their Chosen at birth, so that they do not disrupt their own fate in becoming what they will. The Abyssal Exalted, being corrupted Solars, do not speak to any gods at their Exaltation as such--though they must accept the offer as proposed by their Deathlords, and will later spend time in communion with the Malfeans.
A Spark of Exaltation may not integrate itself into the soul of a person who has no great Destiny — the Loom of Fate must have decreed the Exalted-to-be capable of greatness in his or her own right, although as mentioned above, one who is not born to become a Sidereal Exalted will never become such.
A Celestial Essence may be contained against its natural inclination--the Deathlords use a device called a Monstrance of Celestial Portion, while the Sidereals and Dragon-Blooded constructed the Jade Prison to contain the Solar Essences released in the Usurpation. When accidentally released by the Deathlords, the Exaltation of new Solars began around the world.
An Exaltation resides in the Po--the lower soul of a human being. While this allows an Exalt to accomplish great things with strength and passion and drive, the hungry ghost one leaves behind is nightmarishly powerful. Note that the definition of human can only go so far: Beastmen and Wyld Mutants have been Exalted as Solars and Lunars alike (though the changes harm their ability to wield and develop their Essence, but the Mountain Folk whom the Primordials designed humans in copy of are incapable of this, and the Dragon Kings carry a god's power in an entirely different manner with their ochilike.