I'm trying to follow query guidelines that include a query letter, proposal, outline, author bio, and sample chapters.
What is the 'proposal' part of the query letter? I have not been able to find an answer in the references I've examined so far.
A book proposal and a query letter are two distinct kinds of submissions.
You write a query letter when you have finished writing your book and query whether the publisher is interested in it.
You write a book proposal when you intend to write a book and propose that book to the publisher.
Book proposals are more typical for non-fiction. For fiction, they are usually only written by authors who have already published with that publisher and they both brainstorm together which works would be good to publish next.
For fiction, you commonly write a query letter which should show that you
Query letters and book proposals are two related, but distinctly different tools for selling your book. A query letter is a brief "hooky" one-page document that is often sent as a "cold-call," meaning as your first contact with an agent or publisher you have no previous relationship with. Its purpose is to separate you from the crowd, and to get you a request either for a proposal, an excerpt or a full manuscript. A proposal is a longer document that usually contains a complete synopsis, typically broken down into chapters. Other sections vary, but typically include author bio/credentials, comparison works, sample chapters, marketing plan/platform and target audience. It is usually sent only on request (unless the agent or publisher specifies sending it cold).
Some agents or publishers want you to send the proposal and the query together, others want the query letter first, and the proposal only if they ask. (In fiction, the proposal is often skipped entirely, in favor of a request for the finished manuscript.) There are plenty of general resources available on writing good proposals and many publishers or agents will give their own guidelines.
Proposals are more associated with the non-fiction market, where books are often sold even before they have been written (if the writer already has a good reputation). But they can also be created for fiction–it's a good exercise to write a proposal for your book because it will help you understand how it will be sold. My general rule is that you can always send just a query first, but that you should also have a proposal all ready in the case it is asked for. (If you are doing fiction, neither the query nor the proposal should be sent until after the manuscript is finished, although, of course, you can write them earlier for your own benefit.)
This is my understanding, I don't have a reference for you to look at it.
The "proposal" is either a description of the work you propose to write, or (almost always) a short description of the work (or story) you have written.
I believe actual proposals to represent unfinished work tend to be from either well-known authors that have been previously published, or well qualified writers for non-fiction science, textbooks, etc.
For first-time writers, work almost certainly be done in its entirety, so the "proposal" is generally the description of the story and plot, and no "outline" is included. If I were proposing to write a textbook (to a publisher familiar with the subject and other textbooks) or other non-fiction work for which I am qualified, I would include an outline of chapters.