When you are in Import Job Maintenance on the Data tab, you'll notice the fields are different colors. I could be wrong, but this is my understanding:
Blue - Required
Pink - May be required but not necessarily (may come from a masterfile i.e. Vendor, Customer, etc.)
Red - Optional (I think? May serve a purpose, e.g. LineKey?)
Green - Default to the most common option, usually Yes/No fields.
Black - Although not required, these are usually the fields you want to populate e.g. Amount. (Importing an invoice without an amount is kind of pointless!)
Anyone else care to expand?