Project Plan 3 is designed for members of the Project Managers security group in Project Online, which provides the features needed for project management. Project Plan 5 is designed for members of the Administrators group, which gives them access to every feature in Project Online, including the ability to configure the entire system and to configure the use of the Portfolio Analysis tool. Does that help? Let us know.
The Portfolio Analysis tool in Project Online and Microsoft Project Server is a VERY sophisticated tool, and is intended for organizations that want to optimize their project intake process. Microsoft has developed some fairly good documentation on this tool at:
In their web site, be sure to read all of the articles related to this topic, including how to set up the tool. Also, if you do a Google search for "Portfolio Analysis tool in Project Server", you will find multiple links to YouTube videos on the subject. I have not watched any of these videos personally, so I cannot recommend any of them for you.