It absolutely does not need to be. Just do whatever works best for you.
Any stage of my outlines are just shitposts in disguise. I need to be goofy or else I get bored of outlining, and if I don't outline, then my inspiration for writing dies quick. So I write whatever gibberish gives me an idea of what I need to write.
For example, this is from my scene by scene outline of pages 20 to 23:

It's stupid and short, but its enough info for me. I was able to write a whole lot from this.
Just make sure you can understand yourself. So don't do this:

I still don't know what the fuck this is supposed to mean