Drafts is for your posts, PM, Preview. If you type your post or PM but never click on Preview or Post it, the draft page does not store the draft of your post or PM.
Draft page - Pros & Cons. It only stores your last 100 drafts or 7 days.
If you have draft of your post in draft page, you can copy and paste it in the topic you want to join. If you write a draft for your topic, you should do it on word, notepad and save a file on your computer. When you finish, you can post it on the forum.