9^9 in the Lookup formula is acted as Bignum ( a biggest number that greater than all of your Lookup range value), purpose of the formula is to return the last value in the Lookup range.
Please Google in searching of "excel Bignum", you will find a lot information that could help you to understand.
bosco_yip - thanks so much for the brilliant response to my top 4 from dynamic list of 15 - works brilliantly even if I do not know some of the formula. Don't want to take up too much time but can you let me know what each bit does, particularly the 9^9? Thanks again. Kerry77