What is MOD and when would you use it?

Mod can be used to extend the data, and if the dataset is new it will create the dataset.

MOD is asub parameter which is used to append the data if the data set already exits, or else it will create the data set if doesn't exit. Whenever you want to append the data if the file exists then we can go for MOD

MOD can be used for existing data set (sequential files). It appends data to a file.

mod is a key word which is used with disp parameter as
DISP=MOD
It is used for if the dataset already exit then add the records to dataset at end of the data set.