I've had DO since version 9 and am now quite happily running V.12.
I still have my certificate for V.11 and as it's no longer on either of my computers, (Laptop/ Desktop) so I no longer use V.11, I wondered if I could install it onto my son's computer?
I have a pretty good idea of what your response will be, I can imagine the gasps of breath and clutching of chests as this is read in DO Central Command but I thought that I'd ask.
If you bought a completely new copy of v12 then you can use the separate copy of v11 on another machine, of course.
But if you upgraded from v11 to v12 at a discount, then the v11 licence is null and void as the v12 licence has replaced it. (Opus 11 will accept the Opus 12 licence should you need to drop back a version, but not simultaneously on multiple machines.)
Additional licences are available at a discount if you want to add more machines. We also allow for a "personal laptop" licence free of charge in some cases, although I don't think it would apply to a machine used by another person (but my knowledge, and the forum, are more on the technical side than the licencing side, so I may be wrong).
For more details:
The Licencing FAQ has common questions and straightforward answers regarding licencing.