Originally Posted by Oldpilot
Am I actually creating an OPF file by using the Semantics option? Can I view that file?
No. Sigil creates an OPF automatically (if one doesn't already exist). Setting the semantics only (mostly) creates/modifies entries in the <guide> section of the OPF.
You can certainly view and manually modify the OPF file by selecting it in Sigil's Book Browser.