I think I understand my misundertanding
I was thinking that [H1], ...,[Indent] were Sigil buttons to quickly add the tags. It seems (if I'm understanding right) that the buttons talk to Qt and it's Qt that actually does things to the epub, based on the way Qt wants to do things to the epub
I'd actually tried the <blockquote>\1</blockquote> clip (proud of myself for that

). That does work in CV, but I was thinking the[Indent] button did the same in BV.
The good news (for me at least) is that I spend my Sigil/Calibre time converting PD books to readable (more readable) condition for the family's Kindle. So there's not much complicated formatting, images, tables, etc.
Thanks for explaining
Paul