[KevinH]

Epub3 uses the Nav not an NCX which does not need or use playorder numbers.

BTW, playorder numbers are even optional under epub2, so you could use find and replace on the ncx to remove the playorder attributes and values.

Also under epub3, Sigil does NOT update or generate an NCX at all.

Under epub3 Generate TOC will work on the Nav (as do semantics).

Under epub2 these work on the NCX and OPF guide.

On epub3 Once you have the Nav complete and ready, simply use the Sigil epub3 tools menu to generate an equivalent epub2 NCX and OPF guide entries before your final save. Do not forget to use the Sigil epub3 tools menu to update manifest properties if you have manually added javascripts, remote resources, mathml, svg, etc.