Etiquette is a policy or group of "rules" that help members of society or a group have a pleasant and working relationship. "Good manners" is another way to define etiquette.
"But," you say, "what does etiquette have to do with submitting manuscripts to agents, editors, or publishers?"
Ahh, when a writer (and/or an illustrator) submits his (or her) work, that person seeks to build a relationship, one that will result in his work becoming published. Therefore, a few "rules" should be followed to result in a successful relationship, not one that causes the agent/editor/publisher to reject the work and the submitter with extreme prejudice.
Oh, you didn't realize that word travels between agents, publishers, and/or editors about questionable behavior from submitters? Publishing isn't that large a community. Of course word spreads, good or bad.
Should you build a good relationship, even if your work might be or is rejected? Yes, you want to have a good relationship for at least two reasons: You might want to submit there again; or you want to submit with another house and want avoid a bad reputation.
Rules for good submission etiquette:
1. Research to be sure your work matches what the agency or publisher accepts. No matter how wonderful your submission may be, if it doesn't fit, it doesn't fit.
2. Be sure you understand the guidelines for submission and follow them exactly. No, the agent, editor, or publisher is not being petty to expect you to follow the guidelines. Most are looking for a reason to reject submissions. One that doesn't follow the guidelines is an easy rejection. Also someone who can't or won't follow the guidelines to submit will also be a problem. Yes, guidelines are a test you either pass, or you don't.
For example, if a publisher has 100 or more submissions that follow the guidelines, why would he want to dig through your work, after you don't follow the guidelines, to see IF your work might be worthwhile?
3. Do NOT argue with an editor, publisher, or agent who rejects your work. If you do, you are committing a big no-no. No matter how wonderful you think your work is, telling the "rejector" how wrong he is will not help you win. In fact you will blacken your name.
4. Finally, remember, you need the agent, publisher, or editor more than he or she needs you. Therefore, play nice.
Yes, more suggestions exist, but these are the big ones, the ones that immediately send a relationship into the realm of probability or of impossibility.