What's a primary resource breakdown structure?

A planning resource breakdown structure that is selected as the primary at the project template or project level. The primary planning resource breakdown structure is used for project planning.

When you create a project template, the default planning resource breakdown structure for the project unit is automatically added and set as primary. You can add additional planning resource breakdown structures to the template and set any one of them as primary. Projects inherit planning resource breakdown structures from the associated template. As with templates, you can add or remove planning resource breakdown structures and change the primary designation as required.