Loose Truncation is a new feature that was introduced in Exchange 2013 Service Pack 1. Its purpose is to prevent possible disk space issues that can occur in environments with DAGs when one or more copies of a database is offline for an extended period of time. When enabled, loose truncation changes the “normal” truncation behavior. Each database copy tracks its own free disk space and starts to truncate transaction log files independently if the available disk space falls behind a set threshold configurable by the administrator.
To continue reading, please check the full Exchange 2013 Loose Truncation article at MSExchange.org.