The Metadata Working Group – Specifications:
"The content overlap between the most commonly used standards can result in some confusion. This document describes how best to use existing standards such as Exif, IPTC, and XMP to address the key organizational metadata questions that most consumers have."
An excerpt from the Guidelines for Handling Image Metadata [PDF]:
"A Changer application first reads metadata from the image file and then writes new or modified metadata back to the same file.
The rules for an application in Changer role are:
- It MUST NOT delete metadata unintentionally.
- It SHOULD obey rules for Consumer applications when reading metadata.
- It SHOULD keep all forms of metadata it modifies in sync with each other.
The first rule implies that if the image file contains metadata fields that the application does not understand, it must preserve them when writing new or updated fields back to the file. The second rule
comes from the fact that, almost always, the Changer application is also a Consumer application so it must also observe all Consumer application rules. The third rule states, that whenever, the Changer
application writes new metadata fields to the file, it must keep different forms exiting in the file, e.g. Exif, IPTC-IIM and XMP, in sync. This could also mean to remove a particular metadata
representation while writing another preferred one."