To begin with, I have never seen a copy of ISO 2709, even though the standard is referenced in just about every document that relates to the MARC format. In fact, you often see references to "Z39.2, also known as ISO 2709." Z39.2 is available from the NISO web site, and is the basis for what those of us in the U.S. think of as MARC. So I assumed that ISO 2709 was essentially the same as Z39.2. It turns out that there are some differences that are evidenced in this new standard. They may just be differences in terminology, but here's what shows up in ISO 25577:
- the "Leader" is called "record label" in ISO 2709
- the "control fields" (those beginning with "00") are called identifier field and reference fields in ISO 2709
- what we call "variable" fields in Z39.2 are called "data fields" in ISO 2709
OK, over that hump, the MarcXchange (ISO 25577) is an XML format for ISO 2709. MARCXML is an XML format for MARC21. The difference is the ISO 25577 is much broader than MARCXML. Tags can be anything from 001 to 999 and 00A to ZZZ. And you can have up to nine indicators on a field.
The significance? Well, since you are creating records in XML, certain limitations in the ISO 2709 format do not exist (like field lengths). And you don't have the limitations of MARC21, like limiting tags to 000-999 or having exactly two indicators on every variable field. In this schema, you could create an instance that has no indicators on some fields, and the fields that have indicators wouldn't need to have the same number of them. Think of all of those fields where both indicators have been used and you'd like to add another one. (I don't have the schema in a machine-readable format, but it looks like indicators are limited to one character. I'd love to see that changed so you could have multi-character indicators -- hey, why not?)
No, I'm not advocating that we drop MARC21 for MarcXchange, but could we at least brainstorm on whether MarcXchange could help us out in expanding our bibliographic record where it's needed? No, you couldn't round-trip it, but eventually we have to move forward and quit circling back. Would something like this help us out?