It is as I thought, that for certain / confident to be selected there has to be something substantive such as a certificate issued by the civil authority (city, county, state, federal, in the US) or one of the secondary documents -- and sometimes even that might not be obtained ...
Taken from Types of Sources for Your Genealogy Research
Primary sources are documents, oral accounts — if the account is made soon after the actual event and witnessed by the person who created the account — photographs, or any other items created at the time of an event. Some primary sources include birth and marriage certificates, deeds, leases, diplomas or certificates of degree, military records, and tax records.
Secondary sources are documents, oral accounts, and records that are created some length of time after the event or for which information is supplied by someone who wasn’t an eyewitness to the event. A secondary source can also be a person who was an eyewitness to the event but recalls it after significant time passes.