Why is the document tag automatically added to some rems?

The document tag will be automatically added to some rems and not to other rems.
What’s the criteria for the automatic tagging of the document tag?
I might be missing it but if there is no explanation for this in the tutorials, I think it’s good if this gets explained somewhere.

If you type anything into the source field under the rem title, it will be autotagged as a document. Note that if you untag it as document, the contents of the source field will be destroyed.

Oh boy, that’s such a important point, isn’t it?
It seems like I’ve already deleted some of my resources. Fortunately, it’s not a lot but certainly hurts. I hope others are aware of this.

Anyway, thank you @UMNiK. It helps a lot.