Need to know the expected headers for a source? Pick a source type above and click Download Sample CSV — it gives you a CSV pre-filled with the exact header row that source's parser expects (e.g. regNo,class,chassis,... for CRED, rc_number,registration_date,...).
Tick the files you want. If nothing is checked, the job processes the whole prefix (existing behavior).
Click Refresh to load files from the selected bucket / prefix.