Question:

There is a field called External ID under Base Document Type. It shows the document is in the General section. The field is not visible in the UI on the document metadata section. Why users are not able to see the field in the UI?

Answer:

The document field External ID is a field mostly used in document migrations. It is not intended to be used as part of everyday metadata. This is the reason why the field does not appear in the document metadata section. Hence, it is not possible to make this field visible in the UI. 

Related Documentation:

Vault Help Documentation: Managing Document Fields