it means what it sounds like, you can chose to use either a single 12v rail or you can use it as a multiple rail set-up. check out this review for a write up on the difference https://www.techpowerup.com/reviews/Corsair/RM1000i/2.html
it's really a personal preference which to go with. most are happy with a single rail while others prefer multiple ones. either one is fine with this psu as it is very high quality.