in the editor’s toolbar.Externals are edited in a document-centric fashion.
Each svn:externals property is opened as a new document in its own window and changes are buffered in memory until the user is ready to save them to disk using the File > Save command (key equivalent ⌘S).
Changes in an externals editor can be reverted to the version on disk using the File > Revert to Saved function (key equivalent ⌘U) at any time.
The externals editor provides two interfaces for editing a folder’s external definitions:
The definition list view displays the folder’s external definitions as a vertical list of definitions, with a form provided for editing the contents of each external definition.
The definition list is activated by clicking the corresponding button in the editor’s toolbar.
The definition list is activated by default.
Editing Externals with the Definition List View
The property text view shows the raw text contents of the svn:externals property.
You can use this view to directly edit the property’s text if you are familiar with the svn:externals syntax and need to perform editing operations not supported by the definition list.
The property list is activated by clicking the corresponding button 
in the editor’s toolbar.