Generally speaking, what is a Domain folder? What does it mean/contain?

Lately I have been looking at a lot of different php frameworks and various php packages. Whenever I first look at project, like said frameworks or packages, I try to digest it's directory structure to get a clue as to its organization. I've noticed that there are often reoccuring themes, like an "app" folder, a "tests" folder, a "src" folder, a "var" folder and so on and so forth. Usually I understand what these folders represent or mean, however lately I have been coming across a folder I do not understand in project directories: a "Domain" folder.

What does a Domain folder represent? What kinds of logic or code are contained therein? Does it have any relation to an actual Domain name?

Any and all input would be greatly appreciated.

EDIT: To provide a specific example of what finally prompted me to ask this question, I was looking at the starter boilerplate that is generated by the php "slim" framework. You can generate this project by using php's tool composer and running:

composer create-project slim/slim-skeleton [app-folder]

Here is a link to the repo on github. Within the "src" folder of this project, there is a "Domain" folder. At least for this specific example, what does the Domain folder represent?