It is numbered as the 7.5th installment in the Touhou Project because the events in the game take place between "Perfect Cherry Blossom" (7th) and "Imperishable Night" (8th), although it was released after "Imperishable Night".
Игра вышла после "Imperishable Night", однако описываемые в ней события происходили после "Perfect Cherry Blossom" (7-я часть) и до "Imperishable Night" (8-я часть), поэтому "Immaterial and Missing Power" носит номер 7.5.