Where are defined composite tags ?
Is there a way to define his own composite tags ?
You can find all tags listed in groups on the Tags Names page (https://exiftool.org/TagNames/). There's even a PDF you can download under Additional Documentation and Resources (https://exiftool.org/#links) on the main page.
Composite tags are specifically listed on the Composite tags page (https://exiftool.org/TagNames/Composite.html).
Before asking, I had a look to these files.... but didn't find in them.
With your answer, I searched more carefully... and eventually I found ! Thanks you so much for your reply !
When I'm not sure what group a tag belongs to, I look through the PDF, as that contains all the tag names in a single file. That makes it easy to search for a tag.
The Extra Tags page (https://exiftool.org/TagNames/Extra.html), Shortcuts tags page (https://exiftool.org/TagNames/Shortcuts.html), and Composite tags page (https://exiftool.org/TagNames/Composite.html) are good places to start for tags names that might not be obvious.