Kelley McGrath <[log in to unmask]> wrote:

Like many things, this is not so simple in practice. The meaning of 245$b depends on the preceding subfield (often, but not always $a). Preceded by : it's a subtitle. Preceded by = it's a parallel title. Preceded by ; it's a second title (another work) by the same person or people as the title in $a. In addition, you can have ISBD punctuation within the subfield for parallel titles or more titles by the same person/people.