There are six (6) categories in Trend Micro Email Security under the Inbound Protection Filtering: 

1.  Domain-based Authentication
2.  Virus Scan
3.  Spam Filtering
4. Correlated Intelligence
5.  Content Filtering
6.  Data Loss Prevention

These categories have different Quarantine Reason. For the list of Quarantine Reason, you can refer to this document: Querying the quarantine

Digest Email only makes a list of emails depending on the specified permissions the End Users can view. Therefore, only the selected Quarantine Reason under the six (6) categories will deliver a digest.