JES sets the record format for SYSIN data sets based on the data written to them. If all records that are written are of the same length (before any blank truncation), the record format (RECFM) will be set to fixed (F). If the records vary in length, the record format will be set to V. If carriage control is detected in the SYSIN stream, the record format will be updated to FM, FA, VB or VA depending on whether the records vary in length or not and whether the carriage control is ASA or Machine. If both ASA and Machine carriage control are detected, the record format will be set in the RECFM.