Figuring out how to import GL entries from 3rd party

Where can I find documentation about what an import file for a GL entry requires?  To figure out what data elements were affected, I did an entry and exported it to see all of the data.  There are so many fields beyond what the source system has (it has date, GL account, amount and not much else), how do I figure out what the developer needs to plug and what not?  And do we have to put in each field with blanks/zeros...

Thanks.