DSNONLY: Specify That Only the Data Set Name Is Read

This keyword is applicable only to accessing an indexed VTOC in DSN order.

DSNONLY=NO: Requests that the data set name be obtained from the VTOC index and the DSCB be read into a buffer supplied through the BUFLIST keyword. BUFLIST is required.
DSNONLY=YES: Requests that only the data set name be obtained from the VTOC index. If the ARG keyword is coded, the argument of the DSCB is returned.