SEP Scan - Icon with Tick and a Plus - what does the plus mean?

I'm hoping this is a quick and easy question for someone to answer.

I am just setting up a custom scan in SEP.

If I repeatedly click the folder I want to scan (in the "Create New Scan - Select Files and Folders" window, I get a tick with a plus first, then a tick, then it is unticked.

What does the plus symbol indicate when selecting folders to scan? I thought at first it meant to scan that folder and any subfolders, however now I can see that subfolders are included with just the tick symbol (no plus) so it can't be that.

Thanks in advance for your help!

Adam