you are guaranteed a match on the fandom and ALL the characters you request. (and whatever request you are assigned is guaranteed to fit inside the subset of what you offered.) the person who is assigned your request may not be prepared to write these characters together in a romantic fashion but he/she will have volunteered to write them all. if you ask for, like, four characters and actually just want a fic with Characters A and B, or B and C, or A and D, you can mention that in your details, but otherwise your author is required to use all the characters you formally list in your request. (you are NOT guaranteed a match on anything else, btw; all the other tags are optional, and your author is not technically required to pay any attention to your "additional details".)

hope that makes sense!