Hi
I do not understand the meaning of “Minimum ranking” and “Maximum ranking” when creating a custom game. For example: am I correct in assuming that when I set “min ranking” I am to use the higher kyu value (e.g. 15kyu) while the “max ranking” should then be some strength that is stronger than that (e.g. 12kyu)?? In other words do the entries refer to the opponent player’s strength? 
Yes, they refer to potential opponent’s strength.
An example if you set min. rank. to 15k only players that 15k or stronger will be able to accept your challenge.
By using max rank on the other hand you can limit your game from the other side so only players weaker then the margin can accept.
Since this thread has already been made, I’d like to bring up a related issue: when I try to set the rank boundaries, the drop-down menus don’t appear until I refresh the page and try again. So I have to re-enter all the other details as well, as they revert to default (I know, I should remember to check that the rank boxes work before entering anything else, but it’s not an ideal situation.)