Hi
I didn't find any documentation. Hover over the headings in the columns if you haven't found that already to get an explanation / default values for fields.
I found the the DataImport template from Excel didn't have all of what I needed, so I used that as a base for the GL Account, Customer and Vendor and then used the Edit in Excel from those lists to populate other areas.
Also, another option is the use the Configuration Worksheets - these give more options - particularly for Inventory. In the Data Import, you couldn't classify items as Inventory or Service - I could only get that using the Configuration Worksheets.
Cheers
Heather