Data can only be in format-U for ISO/ANSI Version 1 tapes (ISO 1001-1969 and ANSI X3.27-1969). These records can be used for input only. They are the same as the format-U records described in Undefined-Length Record Format except the control characters must be ISO/ANSI control characters, and block prefixes can be used.

Format-U records are not supported for Version 3 or Version 4 ISO/ANSI tapes. An attempt to process a format-U record from a Version 3 or Version 4 tape results in entering the label validation installation exit.