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Field Description:

The GUID (Globally Unique Identifier), is a string that uniquely identifies the item. When present, a feed reader may choose to use this string to determine if an item is new.

When the 'Is Perma Link' box is checked, feed readers will assume that the GUID is a unique URL for the item, and can be opened to display the full contents for the item.

Field Usage:

This is an optional field. However, this field is strongly recommended for all feeds, including iTunes® podcasts.

Reference:

http://blogs.law.harvard.edu/tech/rss