The select tab is where you can use any of your created or shared rubrics. On the left-hand side you will see a list of rubrics currently available to you, sorted by your personal rubrics and ones shared with you. On the right-hand side is a preview of the currently selected rubric for reference.
You have the option to Select and Attach to the document currently open:
If you are a Google Classroom user, you can also choose to attach the selected rubric to the open document and create an assignment in your Google Classroom class. More information about this process can be found here.