Mana is a magical life energy that allows for humans to activate magic tools, or call upon the gods for their power. The amount of mana one is born with is largely based on the mana capacity of the mother. As one's body grows up, so too does their capacity for mana.
As the heart is most likely the organ in which human's store mana, a person's blood carries their mana inside it. For commoners who lack the means to channel their own mana, they can instead use their blood for the purpose of registering their mana for citizenship or contract magic.
As "moving" mana within the body expends stamina, this can lead to stunted growth within children. As such, noble children are given magic tools that absorb their mana until their baptism, where they'll receive magic tools that can utilize their mana. Upon entering the Royal Academy within the Sovereignty, students acquired a schtappe(wand) to channel their mana. Based on Ferdinand's description, Main believes the organ that stores mana is the heart. When organisms with mana die, such as humans, and feybeasts, their mana will gather at the mana-storing-organ and clump together to form a hardened feystone. The feystones come in various colors and are used in magic tools. When praying to a deity for a blessing, having a feystone corresponding to their color will result in a higher chance of success.
According to Ferdinand, those who lose control of their mana die in a horrific manner (by explosion).
She possesses an extraordinary amount of mana despite her age. Ferdinand asserts Main has more mana than the aub of Ehrenfest and her capacity would only continue to grow as she grows older. The main reason why her capacity has grown so large is due to her repeated compressing of mana whenever it began to overflow, allowing for more mana to fill the gap. This method for increasing one's capacity can be fatal as one would be brushing against death by allowing mana to overflow; adding to another reason why noble children are given magic tools to absorb their mana and are not taught the method until attending the Royal Academy. Ferdinand believes Main has been successful in compressing her mana due to her adult like mind.
In an investigation of Main's mana flow, Ferdinand discovers several clumps of hardened mana, suggesting Main's body had died at least once. When Main gets excited, her mana flow accelerates, but the clumps hinder the flow and her body knocks her unconscious to prevent self harm. Additionally, expending too much mana would lead to a weak flow that is unable to push the clumps, paralyzing her body. In order to dissolve the hardened clumps Main will need to curate a potion based on her mana, which will be a tall task to fulfill.
The social hierarchy within Ascendance of a Bookworm is harshly dictated by mana. Those born with mana from parents who possess mana, are regarded as nobles. Whereas those who aren't, are considered commoners. There are however outliers among commoners with Devouring.
Although possessing mana is a prerequisite for a noble, one must graduate from the Royal Academy to be regarded officially as a noble. Within noble society, mana influences the rank of nobles, breaking them down further into: laynobles, mednobles, and archnobles. If a noble child does not possess mana fitting of their noble rank they will be kicked out of the family; if they're lucky, a lower ranked noble family will adopt them, otherwise the child goes to the temple for the rest of their lives. As such, it's custom for nobles to not announce the birth of their child until the Baptism at the age of 7 to determine their wealth of mana. Succession within a noble family is determined by mana capacity rather than by age.