That feature in 9.15.02 provides administrative control over the power normalization that 9.0.1 introduced. To deal particularly with the fact that due to compliance different channels have different power levels and therefore with ACS it wasn't always possible to predict the cell size, as such with 9.x the controller will cap the max power to the value corresponding to the smallest max value of all the channels in the channel plan. This makes all the cell sizes the same and removes observed issues with coverage holes when customers assumed higher values than what the AP is able to transmit on a particular channel (regulatory compliance).
As this adjustment was causing some confusion in the field, particularly for 8.x upgrades, with 9.15.02 this behavior is now configurable. By default on a new install it is 'on' which as mentioned above normalizes/balances the power level across the channel plan. Disabling the selection will however allow for the selection of higher power values (Note: AP will still follow compliance tables, so it will use whatever value applies to the particular operating channel regardless of a higher specified value in the GUI) and as consequence cell coverage can have different values.
Note2: If ACS is rerun again, it is possible that APs pick a different channel and therefore have a different max power level/cell size (introducing coverage holes or overlaps).
On upgrading from 8.x the feature (Balance Channel Power) will be disabled as of 9.15.2 and therefore the previous power levels will be maintained.