A prose composition in the format of an essay on a topic that has captured the author's interest containing information, ideas, emotions, and/or questions one has about the topic; an expression of momentary inspiration. Any of the usual requirements of an essay or composition may be ignored: purpose, audience, style, organization, the five W's; written for the purposes of fun, relaxation, meditation, exploration, experimentation, or even preparation for a serious composition.
A scribble is more structured than free writing, less structured and less focused than a magazine column. A scribbler knows how to write with a purpose, but chooses to ignore some or all of the rules one must normally follow. Purple prose has no place in a serious essay, but it may have free reign in a scribble. Lies and secrets are welcome to frolick there.
A writeup should not be a scribble. A scribbler should not allow his scribbles to see the light of day without providing a warning to the reader that, though it may look like a legitimate essay, if anything it is a monumentally flawed one. Unfortunately, many scribbles are published every day on personal blogs and social networking sites.
What is the purpose of this writeup? A submission that does not have all the attributes of a good writeup may be called a scribble. Labeling a submission as a scribble is a way of channeling it away from the main content. It's not a draft, because a draft is destined to become a writeup. A scribble is a one-off piece of inspiration.
Why not publish a scribble as a writeup? E2 Users register their opinions on a writeup with upvotes, downvotes, and C!s. A scribble is by definition not a good writeup, so it should only receive downvotes. E2 does not have a 'scribble' flag, but a draft can be flagged as 'public', so that others can read it but they will not be able to vote on it, and it will not appear under any node and mistaken for a serious writeup.
All of this is not to say all scribbles should be hidden in shame. For curious readers, scribbles can provide insight into authors' minds to better understand their other, purposeful writing. But let us distinguish them from proper essays to avoid misleading the readers about the authors' intentions.