How do I properly add a custom source for RSS feed import? I am setting up the mappings for a Feed entity, but I cannot figure out how to properly set custom sources for fields.

In this case, I am grabbing records from an XML document ("RSS/Atom" option) and importing them into an Article content type.

I have a custom tag in the XML document, &lt;alert&gt;, with a value that maps to the Alert Flag Type (field_alert_flag_type) field on Article nodes. I set up a custom source called "Alert" (machine name alert), and when I attempt to import a feed with this new custom source specified, I get the following error(s):

The content [XYZ] failed to validate with the following errors:
    Alert Flag Type (field_alert_flag_type): This value should not be null.

Please check your mappings.


If the XML tag is &lt;alert&gt;, the source name is "Alert", and the source machine name is alert, what am I doing wrong?
The RSS/Atom parsers only work with predefined sources. The custom sources in this case are only useful when using them in combination with Feeds Tamper (or alike).

If you want more control about selecting data from a XML file, try the XML parser from Feeds extensible parsers instead. With that parser, you can select data from a XML file using XPath queries. So you are free to select about anything from a XML file.
It would be good idea to learn more about XPath before using the XML parser. I think that the XPath tutorial from W3Schools is a good resource for this.