Can anyone point me to documentation describing what the not-always-oh-so-descriptive default search schema properties refer to? I'm looking for something like the table here: https://technet.microsoft.com/en-us/library/jj219630(v=office.15) but with a "description" column that explains the relevance of the property. 

Hi ,

From your description, my understanding is that you need a document about the default search schema containing a "Description" column.

I have searched and found there is not a document about the default search schema with "Description" column.

In SharePoint Search Service Application->Search Schema, there are some search schemas which have value in the Description when you edit the managed property. You can use the Description to check the relevance of the property.

Best Regards,

Wendy