Windows Explorer: minus sign in blue box

Self-tutoring about Windows: the tutor mentions something he saw in Windows explorer.

In Ubuntu, under Windows Subsystem for Linux, I was blocked from changing a folder’s contents, even with sudo privileges. The error message conveyed the idea that the file system was read-only, even though the permissions didn’t seem to suggest so.

I went into Windows Explorer to inspect the folder from there, in the Properties tab. Beside Attributes, the box was filled in blue with a minus sign. What was the meaning of said minus sign?

Apparently, the minus sign means that some files in the folder may be read-only.

Interesting, eh?

Source:

answers.microsoft.com