Your sort work file will be variable length only if you define it to be variable length, even if the input file to the sort contains variable-length records.
The compiler determines that the sort work file is variable length if you code one of the following elements in the SD entry:
You cannot use RECORDING MODE V for the sort work file because the SD entry does not allow the RECORDING MODE clause.
Performance consideration: To improve sort performance of variable-length files, specify the most frequently occurring record length of the input file (the modal length) on the SMS= control card or in the SORT-MODE-SIZE special register.