The Report on selection criteria for this report allows you to specify whether the report output should include:
My Primary Projects - The report output will be limited to those projects to which the current user has been defined as a primary Project Administrator association (e.g. Project Manager, Project Lead, Project Approver, Project Viewer, Customer, etc.).
My Primary and Alternate Projects - The report output will be limited to those projects listed above plus those projects to which the current user is defined as having an alternate Project Administrator association or is defined as a global alternate to a user that has access to a particular project.
All Projects I can see - The report output will be limited to the above two universes of projects in addition to any projects that belong to a Project Organization for which the current user has been granted Organization Access visibility (unless the user has the Administrator role in which case no filtering will occur).
Note that the default value for this option will be the most restrictive relationship a user may have in an effort to prevent users from accidentally running a report for all projects. That is, if a user has a Project role, then My Primary Projects will be the default option.
For those users having an Administrator or P&R Administrator role, the default value will be All Projects I can see.