Partial checking mode allows processing only a part of the open source project (notes marked with a checkmark). This makes it possible to process new content without affecting the content that has been already processed; exporting only the desired part of the project, doing a search and making replacements in the necessary records, etc.
Project tree — is the structure of categories and posts of your project. In fact, this is your project because it is a central part of the program. How your project will look at the site depends on how it looks in that tree.
Working with project tree in many ways is not complicated, but the abundance of options and opportunities might confuse a novice to find solutions, that is why this article is strongly recommended to read.