A client got in touch asking how to add rel attributes to links created inside CKEditor.
I could find no documentation from CKEditor or us and found an answer by trial and error.

Mappings are, from labelled field to attribute:
Id id
Name name
Advisory title title
Language code lang
Access key accesskey
Tab index tabindex
Advisory content type type
Stylesheet classes class
Linked resource charset charset
Relationship rel
Download? download

Notes:
Download generates an empty "download" attribute with no value.
Style has validation: "Value specified for the inline style must consist of one or more tuples with the format of "name : value", separated by semi-colons."