Decodable Reader is one of the templates you can use to make a new book or shell book.
A reader is a book that is used for reading exercises.
Decodable readers include only:
words with letters and phonics patterns that have been taught
high-frequency words that have been taught.
Usually, these books start with CVC words (C stands for Consonant; V stands for Vowel). Then later, they use words with more complex spelling patterns. The pictures in these books support the story, but do not help with word recognition.
When you make a book from the Decodable Reader template, it is generally similar to making a book with the Basic Book template. But then you need to set up the stages. See: Decodable Reader Tool overview.
Decodable readers and leveled readers are different.
https://vimeo.com/channels/bloomlibrary/videos/search:Reader/sort:alphabetical/format:detail
https://phonicbooks.wordpress.com/2011/01/28/what-are-decodable-books/