The Settings Tab within the Project Editor is where you can change various settings for the Project and have them be Project specific changes.
Most of these settings are covered elsewhere in the documentation so the links for each subject is below in the Learn About section of this page.
The one setting that is unique to the Project Editor interface is the Item Validation Settings:
When you click this button, you will be able to choose which fields you wish to be notified of when they are blank, or in some cases, negative:
This relates to the Validation Errors column in the Project Editor that will display a yellow warning symbol if an item is added to a Project and it is missing one of these fields. When you mouse over the symbol you will get a list of what is missing from the item:
Most of the settings above when changed within a Project are project specific changes. The exceptions are Project Settings, Item Validation Settings, and Application Settings. These are global changes for your SI 2018 Client.