Both refer to what was called epub "structural semantics". For epub 2 the allowed structural semantics was based on the Chicago Manual of Style. You can still find that term in the epub3 specs but it seems most of the epub 2 info related websites have converted over to only talk about epub 3. (Most websites say epub 2 is end of life and should not be used!).
So I think the term "structural semantics vocabulary" got reduced to just "semantics" in Sigil source code over the years. We just kept it when we took over development of Sigil.
Hope that explains things.
Here is the epub3 spec that uses this term (since about 2010 or so). I can't find the equivalent doc link for epub2 usage.
https://idpf.github.io/epub-vocabs/structure/#
Daisy also uses that term since about 2013.
I think the term "structural semantics" is also used in academic and educational research but I am not sure who first coined that term.