Hello and welcome to the forum,
Why? What system understands with DD DUMMY?
The sustem understands whatever you tell it in the jcl (i.e. dcb info).
DD DUMMY is used to make sure a ddname is resolved even though there is no actual input/output associated with this execution.
Two of the main uses of DD DUMMY are to signify no input (as when //SYSIN DD DUMMY is used with a utility such as IEBGENER) or when the program is to run but there is no need to create a particular output (for example, a very large output report that is not wanted for this run).