All right, I understand very well ! Thank you very much for your brillant explanations.
One last thing, taking the exemple I mentioned first. The Sydney Opera house photo.
I took the photo, I'm the owner of it. I took the photo from a public space, and the opera is now in public domain.
How do I have to fil this form in that case ?
I fill out the form as the owner and the artist, but not sure it suits in that case.
Below, a screenshot on how I fill out the form in my case.
<IMAGE REMOVED BY MODERATOR>

Hi michaelp
Except you are the owner of the Sydney Opera House, you cannot sign as owner. Property owner in this case means the person who has the title for the property (eg land title - land owner) or entitled to the property. If you are not the owner of the property of which you take the photograph of, you will have to find the owner to fill in and sign the owner's section of the form.
You the photographer is the artist. You sign the artist section. of the form.
Even though you took this image from a public space, the Sydney Opera House is isolated. You did not take it as a part of the cityscape, but as an individual building. If you did take it as a part of the city scape as in the example under "Architecture", "Sydney Opera House" at Known image restrictions , then you'd not need a release. Click on "Sydney Opera House to see the example".
I hope this was helpful
Regards
JG