This field retrieves a character through a code value specified in the field-argument. Such documents are common and are created when symbols (e.g. with font Windings through Insert -> Symbol dialog) are inserted in RTF documents in MS Word. 
This functionality is also used when content containing <sym> element is copied from DOCX and pasted in RadRichTextBox - as the content is converted to RTF containing SYMBOL field during the operation.

Workarounds: 
- Implement such field with the API, see the documentation article (http://docs.telerik.com/devtools/wpf/controls/radrichtextbox/features/fields-and-document-variables/custom-fields ) and the SDK example (https://github.com/telerik/xaml-sdk/tree/master/RichTextBox/CustomField/ ). Note that MS Word doesn't show field menus for these fields in RTF documents.
- If RTF format is used and you have control over document creation, save the documents initially as docx, and then save them to RTF. This way the symbols will be encoded without the SYMBOL field construct, and are successfully imported in RadRichTextBox.

Available in R3 2018 Official Release.