In High Reuse mode, incremental implementation is run if cell reuse is above 75%. When cell reuse is below this, placement information is not reused and the flow will continue with the default algorithms. The target WNS is determined by a combination of both the reference checkpoint and the directive.
For high reuse mode, the following message is printed in the log file after place_design has started:
INFO: [Place 46-42] Incremental Compile is being run in High Reuse Mode.