Hi Steve, thanks for your quick replay and your time.
Sorry, I probably did not make myself clear enough (English is not my mother tongue). It is not a conditional statement I was after, I meant anyone editing the post. I have the first part of the code in my main plugin php file, but if I add the second part just after it, it does nothing. That’s why I asked. Should I make it a custom metabox, take it from there and put the whole thing in meta-boxes folder? What am I doing wrong?

Next, I’d like to create a shortcode or a widget to display on front-end a list of that taxonomy. I have tried to follow all tutorials I’ve found, with no luck. Could you please give me some directions on how to do it?

Thanks again for your time and patience.