Karl Voit has an interesting post in which he explores ways of defining projects in Org-mode. Note that by “project” Voit doesn’t mean the type of object handled by an app such as Projectile, but a set of tasks meant to achieve a common goal. If you’re like me, the answer seems obvious: add a project tag to the header defining the project.
Of course, things aren’t that simple, especially for someone like Voit who on the one hand has exacting standards for how things should appear in, say, his agenda but on the other hand wants very fluid Org files. By “fluid” I mean that projects aren’t necessarily top level items or even second level items. They can appear anywhere in the tree. I’m inclined to be more rigid in my Org file layout so I avoid some of the problems that Voit has.
Voit discusses several possibilities along with their pros and cons. I’ll let you read his post to see what solution he finally settled on. If you’re trying to impose a project structure on top of Org, it’s definitely worth taking a look at his discussion.