Hi, glad you are liking the book.
Chapter 12 was a difficult chapter to write, balancing our experiences writing Ecto applications while still maintaining that there is no right or wrong and that there may be other equally valid ways of organizing applications. Where Multi fits best was something we discussed while writing the chapter and it seems like somewhere during the discussions some things were missed.
After discussing this again we are amending this part of the chapter and adding a paragraph that clarifies that we believe multi fits better in context modules than schema modules. The reason being, while multi is a pure data structure like changesets, with multi you usually compose functions from multiple schemas and with changesets you compose functions within a single schema. Since multi is working with multiple schemas it fits better on a higher level inside a context.
Thank you for bringing this up and helping us improve the book during the beta! 
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