Field Mapping provides an interface to match items in your feed to fields in Craft. Depending on the element you've selected to import into will determine what fields you have available to map against.
You can map data in your feed to the Element ID, which is useful if you know for certain an element you want to update. This can be when updating for content in other locales, or bulk-updating existing items.
Do not use this when importing new data. If you're importing your content from another system (Expression Engine, Wordpress, etc), your new content will almost assuredly not have the same ID between systems. This is how Craft's Elements work, and if you specify the wrong Element ID to update, you could be updating the wrong content altogether (an Asset when you meant to update an Entry).
Its important to select a Unique Identifier for your feed to assist with the Import Strategy you've chosen. When comparing against existing entries, it will compare the fields you select here with the data in your feed. Most commonly, this would be the Title for the element, but is also a good idea to be against a unique ID.
You can set a default value for any field, and depending on the field, this may be in the form of a text field or a select field. You can also provide a default value using shorthand Twig (if you enable
parseTwig in your Configuration), referring to other fields in the element you're importing.
Make sure you select the
Use default value in the "Feed Element" dropdown, otherwise it won't work!
You'll see the screenshot above shows three columns. The first is for your Craft field, the second is a dropdown allowing you to select the node in your feed, and the third setting the default value.
You'll notice the
Assets field has a Plain Text row in slight grey below it. This represents a field on the element itself.