If your program uses like sequential concatenation processing, the system calls your DCB OPEN exit only for the first data set and calls your EOV exit for the beginning of each subsequent data set and for each disk or tape volume after reading the first volume of each data set. If your program uses unlike sequential concatenation, the system calls your DCB OPEN exit at the beginning of each data set and calls your EOV exit only for each volume of each disk or tape data set after the first volume of the data set.