Like the left mouse button, the middle button … executes the command indicated by that text. For … then selects with the middle button the word … [or, click the middle button while selecting with the left button]. (By convention, capitalized commands represent built-in functions.) … If the text of the command name is … middle button. Note that 'Cut' is not a … that is bound to some action.
Google-1
Google-2
Copyright clearance needed for quotation.