Layout list
On this tab, you can see a list of all custom layouts created for this web part. These layouts can be selected for specific web parts from their Web part properties dialog under the Layout tab. Layouts in the list can be edited () or deleted (). New layouts can be created by using the Create new layout link.
More resources can be found at:
• | Developer's Guide -> Development -> Web parts -> Modifying web parts -> Customizing web part layout |