Thanks !
No offence but, these commands are so basic that, if you don't know them, it means you should read the manual or watch some video's on youtube about how to start with Eagle.
It may indeed seem strange, now that I see how simple it is, but the most obvious things are not always evident when you learn by yourself in an isolated way.
I have watched carefully dozens of videos on the Web, and read the docs several times, but did not get these points. I have already made several boards without the use of these commands, and probably did much more complicated stuff.
This is the benefit of the Forum. It replaces the possibility to ask a nearby pal for something that may sound obvious, but is only for the ones who know.
Thanks for taking the time to answer.