Hello @autodidaddict, I think that “notifiers” near the end of this sentence:

As you’ve learned the building blocks in this first part of the book, you’ve picked up a few rules worth going over again to help you understand why notifiers and projectors exist.

should be “aggregates” instead:

As you’ve learned the building blocks in this first part of the book, you’ve picked up a few rules worth going over again to help you understand why aggregates and projectors exist.