The definition of metafiction is a book using fictional elements to talk about writing or reading fiction. Some examples of metafiction are Little Red Writing and The Plot Chickens.
The definition of nonfiction is a book that teaches concepts or in large part sticks to the facts.
Sometimes nonfiction has fictional elements to help make the concepts more interesting, such as a talking bookworm or school buses lined up inside a whale. Other fictional stories are slated as nonfiction because of their classic status.
Sometime the metafiction label is attached to stories that encourage interaction from the reader, such as Press Here or The Monster at the End of This Book.
If you aren’t sure whether it’s nonfiction, fiction, or metafiction my suggestion is:
Don’t Label It at All.
Instead, mention other books that your book is like and let the publishers and librarians decide where to put the book.