To identify new or updated records in ServiceNow is a straightforward way. However to find the records deleted in a certain time window is not so simple. Most of the tables are audited so the default strategy search for deleted records in the system journals. You can choose different strategies if you need or if the table is not being audited. Truncate option means to perform Clean & Synchronize operation for every sync run (initial load every run). Diff is a good way to find the deleted records in a non-audited table and None ignores the deletes at all.
Please read more about the delete strategies in our user guide.
Please note that synchronization settings for Views do not contain the delete strategy choice at all. It always performs the Truncate option because there is no way how to achieve incremental updates.