what is reference even best answer questions are there in it?

A reference question is a resolved question. Which means best answer has been chosen or voted. You can still answer it but get no points for answering, and, obviously, answers given at this point can not be considered for best answer. Why? Because there already is a best answer. We don't answer to express our opinions of previous answers. Either provide another answer or don't bother.

when Y switched to this new format, they simply changed the name of a closed question from Resolved to Reference

the only difference now is that you can leave an answer to a Reference question, however, you do not get any points for doing so

BEWARE - you cannot edit or delete an answer you leave on a Reference question so be sure you want to answer and that you have checked your answer before hitting Submit

keep in mind that very few people look at closed questions so not many are likely to see it

for that very reason no one is really sure why Y bothered to make this change but apparently some suggested it

also keep in mind that answers to Reference questions must follow the Community Guidelines and they can be reported if they don't which will cost you ten points