What is the difference between SelectionInput and FormInput?

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I currently use the FormInput on my Customer Portal so they can edit the Severity of the Incident. I wanted to remove the "'--" value and in another forum, question they said to extend the SelectionInput but I use the other one.

I placed both on my site and they look exactly the same. I've also looked at the documentation for both and it is not very clear.

If anyone could explain the difference more clearly I would really appreciate it.

Thank you.