bold is the old way (and older readers want that as well as EPUB2
)
the numbers are the newer way of specifying font-weight (I have seen a chart. there are more number levels than the old)
Each font has an internal table (normal, bold, italic, bold-italic). That is 4 font files to make up the complete FAMILY (not al faces have all 4, so substitutes are sometimes called out and the check will complain that the names do not match the
Family