Overview:
Not able to filter a document relationship search by Sub-type or Classification.
Root Cause:
The Type is not selected when setting up the Document Type filter.
Solution:
To set up a Document Type filter for a document relationship search use the following steps:
- Search for the proper Document Type.
- Press the Space Bar key after the proper document type is entered.
- Type the > character.
- Type the first letter of the desired Subtype in order to see the desired Subtype and its Classifications:

Related Documentation:
Vault Relationship Documentation: How to Add & Remove Relationships