intellij-idea - What is the 'Resource Root' designation for in PHPStorm? For that matter, what does PHPStorm consider to be a resource?

Question

Maybe I'm being thick, but I fail to understand what the 'Resource Root' button/designation is for when creating a project in PHPStorm. Having looked at the PHPStorm documentation on the topic, the only explanation I've found is a variation on the following:

"Resource Root: By assigning a folder to this category, you tell PhpStorm that files in it and in its subfolders can be referenced relative to this folder (instead of specifying full paths to them). These folders are marked with the icon [purple directory]."

That doesn't answer my question.

Answer 1

I can answer anecdotally that URL paths will be resolved against "resource roots" where applicable. For instance, in HTML <a href="..."> or <img src="..."> elements, relative URLs are looked up against resource roots. When you start typing within the href or src attributes, auto-completion will suggest items in those directories, and URLs which cannot be resolved will helpfully be marked as a warning.