One explanation of workflow is the (optional) scripting ALM allows to modify the logic behind the GUI. Workflow customization is available for all modules, much more on this in the documentation for sure.
With workflow you can modify the default behavior of nearly any element of the GUI including what list values you see to what happens when you save a record – the possibilities are pretty much wide open.
In your scenario the click on the ‘Generate’ button could be ‘short circuited’ to run a custom function instead of the default behavior (see ActionCanExecute) or more useful might be to add your own toolbar button (next tab over from workflow customization) and using this to call a custom function prompting the user for input, grab data from the current record in the UI, call a database query, export a report, etc. The workflow script is pretty much open to anything you can dream up, just not much available for changing the GUI itself.
VBScript (VB 6 style for the most part) is used in the workflow. Again, a lot of examples are in the docs. I mention the workflow option primarily because it keeps the code in the project so that’s handy and a lot of other reasons. Consider that customization of the project isn’t supported, but neither is scripting in OTA or REST but at least workflow script in the project is one less external app you have to track and maintain to get the job done.
OTA or REST Api's do open up a lot of possibilities but carry the burden of supporting them outside of ALM. I'm not sure what the right balance is for your situation of course.
