If you order online from Disney, it asks for your email address. If you have a
MDE account affiliated with that email address, the tickets go into that MDE account automatically.
If you order from a place like
Undercover Tourist and they mail you the tickets, you can enter the tickets into your MDE account.
Once a ticket is in your MDE account, you can assign it to someone in your MDE account. After you have assigned it, you can re-assign it to someone else in your MDE account if it has not been used.
Once a ticket is in your MDE account -- I don't know how you give it to someone who is not in your MDE account. I believe there is a way to do it if the ticket has not been used, but I don't know if it requires a phone call to Disney tech support.
If you order from a place like Undercover Tourist, they mail you the tickets but you have not yet entered the tickets into your MDE account - you can give the tickets to someone else.
I'm sure someone will jump in if any of this is wrong.