I've just made my first booking using Sabre, and when looking at the e-ticket receipt on virtuallythere.com I see the following line:

I presume the first part means that I can't endorse the ticket over which comes as no surprise, but what does the "no upgrades" mean specifically?
I can't find this phrase in the fare information, and while BD don't do upgrades with miles or anything yet, I was hoping to use one of my gold upgrade vouchers to upgrade to premium economy.
I don't think that I've seen this phrase before but I may have just overlooked it. I wouldn't have thought it would affect my chances of using the voucher, but would just be interested if anyone knows what this line on the ticket specifically means. Probably a stupid question I know.
Thanks