The point is that "RUC" is not a term used internally at Delta - they call it an "Upgrade Certificate". You might run into a rep who can noodle out what you mean, or one who has heard it in the past and knows what you mean, but if you get someone who is not familiar with it and they search their documentation for that 3-letter acronym, they will come up empty.

I'll give you a semi-related example - the company that I work for is a dealer for another manufacturer's printer. They have a piece of software called the "Printer Control Utility", which as names go, is simultaneously very specific and kind of vague. Our customers sometimes forget which tool to use when we are providing phone support and direct them to launch the Printer Control Utility. But the icon that the manufacturer uses for the program is very distinctive - it's a rainbow in the shape of a donut. My team of trainers have taken to calling it "the Magic Donut" - it's easy for the end user to remember and associate. But if the customer called the machine manufacturer's support line and asked for help with the Magic Donut, they would probably be put on hold until the agent could maintain a straight face.

We can't expect them to know terminology that we make up.