-D or -d does nothing in set command.
In fact here in your line -D is not a command line parameter or an option.
above command will save "-D_ABC -D_DEF" this string as an envirnment variable VAR.
thats all.
How to use this is up to the user who created such variables.
set is a command to set dynamically the environment variables but its scope is only till the current cmd is opened as soon as cmd is closed its scope will be lost.
Be carefull -D is used to define the system property in case of virtual machine.
But here it doesn't seems so please look into the use of set command .
Comment