Taiyouchuu (a Touhou LW original) refers to the light pillar that sometimes appears above the sun at sunrise and sunset. Mokou fights with her body enveloped in flames, so this temperament is a perfect match for her. Mokou is endowed with Fire Phase, from the intense emotions swirling within her, and Sun Essence, from the strong will she uses to control those emotions. The formless and passionate Fire Phase resists Wood Phase, because wood feeds fire, but it is weak to Water Phase, because water dampens fire. The alluring and arrogant Sun Essence resists the diverse and uncooperative Star Essence, but it is weak to the cooperative and indecisive Moon Essence.

Just as the ability says, Fujiwara no Mokou is eternally young and immortal. This is the power of the people of Hourai. If you take the Hourai Elixir, your soul becomes immortal, so even if your body is completely destroyed, your soul becomes your core and completely regenerates your body. In other words, the life of a person of Hourai does not depend on their body at all. You could even say that their life transcends that of an ordinary human. That is the wish of all humans everywhere, but for Mokou, the moment she obtained this ability and the hardships of her life to come only made her feel fear and loneliness, and she had a long period where she was overcome with despair. That she can live positively as a regular girl now even with that kind of past is probably due to Gensokyo's accepting nature and the relationships she keeps.