That'll teach me to play - I think I've found a bug!
There appears to be an issue with Right-click-on-filename>Add Semantics - it appears to be toggling the semantics rather than setting them and not telling you what it's doing.
For example: on a clean start of Sigil file "Section0001.xhtml" has no semantic set and the <guide> tag in content.opf is empty
If I set the file Section0001.xhtml to have a semantic type of "text" the correct entry is created in content.opf
Code:
<guide>
<reference type="text" title="Text" href="Text/Section0001.xhtml"/>
</guide>
If I repeat the exact same process and apply "text" to "Section0001.hxtml" again the entry in <guide> vanishes.
This is obviously a new meaning for the word "Add" that I previously wasn't aware of
Not realizing this was happening initially caused no end of confusion!
Andy B.