http://www.idpf.org/epub/20/spec/OPF...htm#Section2.4
In the ePub specifications, in "Spine", you can define if a content is auxiliary, not displayed/skiped by the Readers, but accessible via a link elsewhere in the text.
This is defined by the property
linear="no", but I can't find anything in Sigil for this feature.
Not implemented? or where else?