Beginning in 2018 all bats that are greater than a “drop 3” / “-3 ” (-5, -7, -9, etc.) will be ineligible for use in league and tournament play unless they have a new YBBCOR certified USABat licensing stamp. Eligible, YBBCOR certified bats are expected to be available for sale in September 2017.
“Drop” refers to the difference between the length and weigh of a bat — a 30 inch, drop 5 bat will weigh 25 oz. Drop 3 bats will still be allowed in 2018 as long as they are BBCOR certified.
If you are interested in reading more about the USABat standard, you can find additional information here.