When selecting Filters for the Vault Library, an Object Field cannot be selected without another Field being displayed along with it in the Filter.
The second Object Field displayed for this Filter is set up as a Child Object of the first Object Field. This allows the user to select a related Child Object record for this Filter in order to narrow down the displayed Library results further if desired. In the below image, for example, Sub-Product is a Child Object of the Product Object, and both Objects are set up as Document Fields.
Vault Help Documentation: Configuring Vault Objects
Send us your feedback: We're always looking for advice to help improve our Knowledge Base! Please let us know if this article was helpful or provide feedback on how we can improve your experience here.