I'm signing up for an Amtrak Guest Rewards account, and I see at the bottom a pop-up menu of "Enrollment Codes" (shown on the next page--the confirmation page--as a "Promotion Code"). You can leave it blank, but you can also choose from among several codes, from SAF04 for "Student Advantage Offer" to DRC05 for "Diners Club Bonus."

What I can't figure out is a) what they're there for and b) which one I should use.

I am a Student Advantage member, so should I choose that one? Or should I randomly pick, say, the Cendant promo or the Morgan Stanley one? Why on earth would they just let you choose a promo code?

At no time does it actually ask for PROOF of the qualifications for the promo, such as a Student Advantage number or anything.

I did find a reference to the Diners promo--a PDF on their website (search that promo in Google) said you can get 1000 points if you're a Carte Blanche holder. (How do they verify that if they don't ask you for a Diners Club number?). I searched to the Student Advantage one, since that's the one I qualify for, but I didn't find anything. I didn't spend time searching for any others.

Any ideas?