Thank you for quick answer,

I have changed the related line as you said; <h:inputTextarea value="#{currentDocument.contextData['request/comment']}" required="true" />

That worked, "Save" button doesn't allow me to make a version increment without adding a comment. But there is no sign about this field is mandatory and if you click "Save", no information about the reason of failure. I was thinking a change like a red * icon too as "Title" field has. I have to give at least one of them to users (red * or information about that field is mandatory).

Another thing is; my edit is not permanent, whenever I restart/force-reload the nuxeo server, the line returns to it's previous version.

For the red * to appear just change the class on the "td" element around the label: class="labelColumn required"

And changes to xhtml files are not persisted: you have to override this template in your own plugin.

If you made the change to the XHTML file on the running server then the next time the server stops/starts the nuxeo WAR will be redeployed and erase your changes. To make this permanent you need to make a contribution to override the default document_edit.xhtml file.

Ok, I think I got how it works. The rest is not problem. Thank you so much.