The report source you choose determines the available fields for building reports. The fields are presented in the Available Fields panel when you are defining your report output options.
The Primary entity is always the top folder. This entity drives the relationships to other entities based on its position in the PPM Pro entity hierarchy.
Common fields (and All), directly on the entity
Related fields are from entities that have 1:1 ratio with project
Children fields are from entities that have 1:n ratio with project
Example: use Related/Resource folder if you want information on the Project Owner. The Project Owner field is connected to the resource record for the owner. So if you want more information, like the project owner's email address, you have to get it from the related resource fields.