I want to add metadata to plain text and binary files. These files tend to be derived from a canonical source on the internet (which might get updated etc). For example, I might do something like curl $URL | obscure_magical_filter_that_took_me_ages > file.txt
If possible, I want to do this in a vaguely standard way. XMP would appear to be a good standard, with exiftool being a good command-line tool for interacting with this.
I can create sidecar files like so:
exiftool -Creator="Joe Creator" -xmp -o test.xmp
The issue is that there doesn't seem to be a metadata field for "how was this made?". What should I use?
Exiftool's documentation has a good list of fields. Out of these, OriginatingProgram seems like the best match: Is this a good thing to use?
Where can I put metadata for "How was this command created"*.
There really isn't a specific tag along these lines that I can think of, so you can either re-purpose a tag or create your own tag.
One thing to take into account is how are you going to access the info in the sidecar files. OriginatingProgram is an ACDSee specific file and is unlikely to be read by most programs that would read sidecar files. Additionally, it sounds like it would likely be overwritten if ACDSee ever changed data in that sidecar file.
If you're setting up you're own system to read and processes sidecar files, then creating your own tag might be the way to go. Exiftool's example config file has info on doing so and more help can be found on the exiftool forums. But such a tag will probably not be visible to most software.
Re-purposing a more common tag that you might be a convenient strategy. If you have no plans on using a tag like Title or Description, then those are easy and well supported tags. Otherwise you could look to re-purpose a different tag that may not be relevant to the file, such as City, State or Country. Instructions might also be a good choice, as I believe it's intended purpose is for restrictions or right usages, which wouldn't really apply to a non-image or video file.
Personally, I tend to place miscellaneous info that is still important to the file in HierarchicalSubject. You can set it apart from other keywords in it's own hierarchy and it's still available and easy to see with most software.