Hello -- the answer is in 7.2.3 of 3GPP TS 23.140, v5.3.0:
"Additionally, it shall be possible to control which recipient(s) address(es) are utilized for actual routing and which are conveyed as informational only to be displayed to the recipient MMS User Agent."
So any "displayOnly" addresses should NOT be used for routing -- they just show up at the far end as recipients.
Now the question is: what is the point of that? Got a good answer? I'd love to hear it. The sample message submission in 220.127.116.11 of that same spec doesn't really shed any light as far as I can tell. They send the message to 7255441234 and 2 other addresses, but additionally list 7255442222@OMMS.com as a "displayOnly" recipient.