[KevinH]

When the external XHTML is set to PageEdit and launched by using the XEditor icon when an xhtml file is being edited in CodeView, it will create a parameter list that specifies a path to the opf file, a spine order number that specifies the xhtml file to be edited, and with the latest versions of Sigil, with add a file cursor position offset to the passed parameters. Sigil then knows to set file watchers on all of the epub's xhtml files so that Sigil knows when and if an xhtml file has been changed by Pagedit and reload them.

PageEdit uses this passed in information to build the list of xhtml files in spine order and identify the specific xhtml file you want to edit. And it will even try to match cursor position from CodeView.

This matching of cursor postion and the setting of file watchers on all xhtml files is only done when using the XEditor, not with Open With. So they do differ.

It does not edit or change the OPF, it just uses it as a way to pass all the paths to the spine's xhtml files including the one currently being edited in CV.