A very good question italki's explanations are ambiguous and are the reasons people can't understand the difference, especially between notebook and answers. The English language is not clear, I can not speak for other languages.

But it seems you just post a discussion into the discussion section for people to discus your topic.

You write notebooks that people can correct a bit like doing homework or an assignment from your teacher. Italki gives recommendations on the things to write about.

The Answer section is where you ask questions on anything such as what does this word mean I have never heard it before and people will mostly reply that is a slang word in country X or an Idiom.

You are right it is difficult for a language learner not to be able to pick out these subtle differences when they are still novice learners. It is not the novice learners fault.

For example on each page about these sections there is written text that contradicts what is written elsewhere on other explanation pages. It seems to be a case of the copywriters forgetting what was said on another explanation page and not making the various different pages agree.

The Discussion page is for having a discussion, inviting many people to give an answer or thoughts. Many people abuse it, and use it to say they are looking for a partner.

The Answers page is for simple and quick answers, and usually only 2 or 3 people answer each question.

The Notebook section is if you want to write much more, e.g. half a page of a story/diary, and have someone correct your grammar and choice of words.

I don't read every page, mostly just the Answers and Discussions pages, and ignore all the partner requests, but other people will respond more.

@Phil Yes, I saw. Thank you, Phil

Hi, Lee! Are you sure you're a novice? Anyhow, Miriam has posted excellent good discussions that will answer your question. You can follow the links in her comment below.

Lee: Welcome to italki! While it’s better to post texts for correction in “notebooks” and texts for discussion in “discussions”, it’s also possible to post a discussion and mention that you’d appreciate corrections. Otherwise, it generally comes off (is perceived) as off-topic to provide corrections on discussion posts.)