With SMS, the SPACE parameter overrides the space allocation attributes defined in the data class for the data set.

Explicit specification of SPACE on the DD statement overrides both the SPACE and the AVGREC values specified in the data class.

One exception is if a data class with OVERRIDE SPACE(YES) specified is assigned to the allocation. In this case, the space allocation attributes defined in the data class will override what is specified on JCL even with explicit specification of SPACE on the DD statement.