I'm sure Bob Stepno won't mind if I use his question/suggestion as an example of how outliners are different from most other programs.
On our Q&A page, he asked how do you select-all in an outline. He was disappointed that when you press Cmd-A it only selects the siblings at the same level as the bar cursor headline. He felt it should select all the text in the outline.
My answer was that like almost every operation in an outliner, select-all is scoped. When you select a headline, you are implicitly selecting all the headlines that are subordinate to it. So if you want to select everything in an outline, move the cursor to one of the summits, and do the select-all there.
This is fundamentally different from the way a word processor or spreadsheet works, but it's exactly the way the Finder or Windows Explorer works, which are also scoped. When you select-all in a folder, it doesn't select all the files on your computer -- it just selects the files in the folder you're working in.
Why does it work this way? Well, an outline can logically contain many documents, not just one. For example, I'm writing this blog post in an outline that contains all my posts dating back to March of last year. If I did a select-all here, I'd want it to select all the text in this post, not all the text in all posts. However if I wanted to select everything, I could do that, by putting the cursor on the top level headline, as illustrated in this screen shot.