From the text (p. 6):
Worlds Numberless and Strange offers detailed glimpses into dozens of recursions, giving the game master all she needs to send her PCs for a brief visit to any one of them, for NPCs to slip from them to visit the characters on some other recursion or on Earth, or for the GM to use them as the basis for more in-depth setting material of her own creation.
From the text (p. 7):
Recursions are unique but limited worlds hosted in the dark energy network of the Strange. They are not parallel worlds. The concept of parallel worlds was popularized by the “Many Histories” or “Many Worlds” interpretation of quantum theory. The limited alternate worlds of the game, called recursions, are created by fiction, not branching possibilities. Novels, myths, movies, and even urban legends seed themselves into the Strange, and implausible new locations are born.