Double-check all your entries, then click Apply to create the new partition.
You’ll get a warning message, read it, then click Partition.
Because all of my media and projects are stored on an external drive, dual-booting allows me to select which version of Final Cut Pro X I want to use for an edit.
NOTE: Actually, you can create any number of different boot disks, to run whatever software you want.
NOTE: The name of your hard disk – which will probably be a gibberish of letters and numbers – will vary.