Version 4 allows user data written in any national or user defined character standards. The character standards are specified by the CCSID parameters on the JOB/EXEC and DD statements. For details, see z/OS DFSMS Using Data Sets. If BUFOFF=L is in effect with CCSID conversion, the access method does ASCII conversion on the block prefix and CCSID conversion on the rest of the user data. The Version 4 standards require this distinction.