I've never heard this term... but knowing the actual question from the sequence might help in being able to determine what they are. It could be prime numbers taken in reverse...? (ie, 13/31, 17/71) Or otherwise it could just be prime numbers taken in a reverse order. (17, 13, 11, 7, 5, 3, 2)... but I'm not really familiar with the term myself. So I'm quite interested in the solution to the problem if anyone else might know.
If not, the answer to the question might be able to explain what they are.