When you select a project or task (in Change Attributes, the attributes that are defined in appear in the dialog box, along with the project system attributes that you have defined (for example, Due Date, Cost Model, Client, Project Type, and Quality Model).
) and clickChoosing Select All will modify all attributes for all selected projects. Only do this if you want to change all attributes for all projects.
The Change Attributes dialog for a project (Change Project Attributes) differs from that for a task (Change Task Attribute) in that the former lets you change system attributes, whereas the task dialog only lets you change custom attributes.