What is the meaning of this icon in Eclipse?

Question

I'm using the Eclipse IDE Mars and I made a maven project. I did dependency setting, file generation, code writing, and so on. At first, it was fine, but at some point, the error 'maven java e configuration process - an error occured while filtering resources' appeared in the project. So to solve the problem, I clicked on the project and clicked maven->upgrade project, and the error disappeared, but the above icon appeared. What does this icon mean?

Answer 1

It's simply a feature of Eclipse where everything that belongs to test sources is visualized with darker icons.

Classpath separation via test source prevents JUnit and other test libraries and code from being used accidentally in the main code (src/main/java).

Since Eclipse Photon, in Java Maven projects source folders containing test code (src/test/java) are marked automatically as test source.