FictionJunction is the solo project of Japanese composer Yuki Kajiura featuring guests musicians, mainly singers. Historically, works between Kajiura and a vocalist were referenced by adding the singer's name after "FictionJunction". Although both perform under the FJ title and are collaborations with Kajiura, they are separate projects.
Source: “Yuriko Kaida”, English Wikipedia (CC BY-SA 4.0), excerpt