If you are focus is about the flat files then one can set it in file properties while creating a mapping or at the session level in session properties
What are mapping parameters and varibles in which situation we can use it
If we need to change certain attributes of a mapping after every time the session is run, it will be very difficult to edit the mapping and then change the attribute. So we use mapping parameters and variables and define the values in a parameter file. Then we could edit the parameter file to change the attribute values. This makes the process simple.
Mapping parameter values remain constant. If we need to change the parameter value then we need to edit the parameter file .
But value of mapping variables can be changed by using variable function. If we need to increment the attribute value by 1 after every session run then we can use mapping variables .
In a mapping parameter we need to manually edit the attribute value in the parameter file after every session run.