This inspection warns you of characters that current encoding is incapable to represent.
E.g. when you are trying to type international characters in an US-ASCII-encoded file.
Typically, you would fix this by changing the file encoding, either by specifying the encoding directly in the file, e.g. by editing encoding= attribute in the XML prolog of XML file, or configuring the Settings|General|File Encoding|Default encoding setting.

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