Many of the actions in Setup Factory produce results that must be stored in variables. For example, the Read from Registry action reads a value from the Registry, and assigns that value to the variable of your choice. The value that is read from the Registry is the action's result, and the variable is where that result is stored.
Each of these actions has a field where you can provide the name of the variable you want the result to be stored in. If you provide the name of an existing variable, the result will overwrite that variable's contents. If you provide a new variable name, a new variable will be created automatically. The action's result will be stored in the variable you provide, regardless of whether the variable already exists.
Essentially, wherever a result needs to be stored, you can create a new variable "on the fly" by simply providing a variable name that isn't already being used in your project.
See Also: Variables, Defining Variables with Screens, What Can You Do With Variables?