The Data Substorage Key value is used to identify the data store you're creating or reading from.
Write variables by pressing the plus button, and defining key-value pairs.
Read in variables by pressing the plus button and then defining the key name of the variable you want.
You can also define a default value to assign to the variable if it is not found in the data store.
Variables read in are created, and can be found in the variable section of the interface.