Creating a new extension is covered in detail HERE .   After you click on the "Add New Extension" button on the creation screen, you will arrive at the extended options screen.  This is the same screen you will reach if you use the "Modify Existing Extension" tool.  There are four navigation tabs at the top:  General, SIP & Dialing, Forwarding, and Advanced.

The General tab contains most of the data that was configured when you created a new extension, with some additional options that were not available in the creation process:  Line Button Label and Zero-Out.

Line Button Label

The text you enter in this field gets automatically added to the extension provisioning file.  This only applies to phones that you are auto-provisioning.  If you are manually provisioning the phone for this extension, this text field will have no effect.  For auto-provisioned phones that have LCD displays, the Call Appearance (Line) buttons will be labeled using the text in this field.  If you do not enter text here, the provisioning generator will use the text entered in the Name field.  Since the number of characters available to display on a phone screen is limited, a person's full name will often be truncated when it appears on the phone.  This label field allows you to use a shorter, alternate text identifier - perhaps the extension number or a short name.

Zero-Out Option

This option, labeled "After Abandoning Voicemil, go to:", is for configuring the option for callers to redirect to another destination if they reach a user's voicemail.  Pressing "0" during the voicemail greeting will send the call to the desired target - either an available auto-attendant, or a call group.  If you deploy this option, you should instruct the user to notify callers in their voicemail greeting recording that this option is available.