I’m afraid I didn’t clearly state that the most reliable way to edit metadata fields is to use whatever program created the file in the first instance. DT does not have the ability to access every metadata field that a given type of document might have, and in many cases can only display the field content.
These metadata fields are the ones you see in Tools > Show Properties, namely “Author”, “Company”, “Copyright”, “Title”, “Subject”, “Keywords”, “Comment”, in addition to the ones you see in the dropdown list used to chose column headers, namely: “From”, “To”, “Recipient”, “Headline”, “Description”, “Subject”, “Organization”, “Copyright”, and “Album”. You might recognize that many of these fields are from iTunes. Others are from the image metadata standards EXIF and IPTC. And others from the PDF standard. These are not the only file types that use these fields, but I don’t have the time or space to list all the culprits here.
DT does a better job of displaying metadata than editing it. AFAIK, the only file type for which you can reliably edit every field in Show Properties is RTF.
But, remember that tags are also metadata and are infinitely more flexible and content-rich than the metadata mentioned above. You can accomplish the results you mention using tags.
Another very excellent approach is to use the annotation templates – look in the forum for Bill DeVille’s meticulous descriptions of his approach to using them. In many ways Bill’s approach is merely another form of metadata, but far richer than the static metadata fields that are available on a limited basis for a limited range of documents.
@nestor - the editable field you can edit is now called “Spotlight Comment” and is always editable when it is a displayed column in a view, and in the Info panel. See my original response, above – the field that cannot be edited that way is “Comment”. (The change that made a distinction between “Spotlight Comment” and “Comment” was the release of DT version 2.)
@nigeldgreen - very very few Mac programs will display Spotlight Comments. They are, however, always searchable in Spotlight.
Sorry for the very long responses. Unfortunately, when it comes to metadata fields, confusion often reigns.