When trunc log on chkpt is true (on), the transaction log is truncated (committed transactions are removed) when the checkpoint checking process occurs (usually more than once per minute), if 50 or more rows have been written to the log. The log is not truncated if less than 50 rows were written to the log, or if the Database Owner runs the checkpoint command manually.
You may want to turn this option on while doing development work during which backups of the transaction log are not needed. If this option is off (the default), and the transaction log is never dumped, the transaction log continues to grow, and you may run out of space in your database.
When trunc log on chkpt is on, you cannot dump the transaction log because changes to your data are not recoverable from transaction log dumps. Use dump database instead.
By default, the trunc log on chkpt option is off in newly created databases. To change the default, turn this option on in the model database.
WARNING! If you set trunc log on chkpt on in model, and you need to load a set of database and transaction logs into a newly created database, be sure to turn the option off in the new database.