Source view in a value explorer presents the value as the source language (AppleScript) would present it.

For example:

• In Best view, an object reference is an outliner. In Source view, an object reference is the reference.
• In Best view, a list is shown as an outliner. In Source view, a list is delimited by curly braces.
• In Best view, a string is shown as the text of the string. In Source view, a string is delimited by quotation marks.

You can show more than one view at once. To do so, hold Shift as you switch views. The result is a split-pane display of multiple views.