Answer
Use the External File Assistant's 'DescribeInput' function for the specific file type you're importing, choosing the Online option and working through the relevant describe-input steps — this generates a fully readable, human-friendly explanation of exactly what record types and columns that specific input file type expects, rather than needing to reverse-engineer the required format from trial and error. Running DescribeInput against your target file type before attempting the actual import is the reliable way to confirm which record types are required, which columns matter, and how each column maps to its destination field.