Named difficulty tiers are the spec: each tier defines what its level means in generator settings, with an optional gentle drift from start to end across the batch. One tier + one theme list = one single-level book's run ZIP, ready for Book Builder.
Theme list
No list loaded.
Tiers (word search)
Start of book
End of book (with drift)
Numeric settings blend puzzle by puzzle; directions and fill switch at the midpoint.