The Audit Trail Classic table in your SQL Server database (zd_Audit) is your to do as you like with. Audit Trail Classic does not have any requirement for there to be data in that table in order for it to work. Of course if you delete historical data then you will not be able to see changes to records that have happened in the past.
At some point though it may be you have decided that data of a certain age no longer need be retained. You are able to use SQL to move the data or delete the data from the table. However if you are not familiar with SQL then there is a tool built into Audit Trail that allows you to do this.
Log into Raiser's Edge as a supervisor user. Go to the Audit Trail Viewer and press the Z logo to bring up the about box:
Select "Delete Historical Transactions"
Select a date range. The count will automatically update to show you how many records you will be deleting. Please ensure that you have a back up of the database in case you regret your actions.
Press OK to delete and the transactions will be deleted from the database.