Generally, you go with the Public offering if you want your Q&A to be accessible anonymously, and you want anyone to be able to sign up and contribute. You may want one of the Private plans if the Q&A is to be accessible to a defined community only, either inside your organisation, or between several organisations. As for On Premise, people usually choose it for reasons of special security constraints, or integrations with behind-the-firewall systems like users directories.

Visit our Use Case page for more details about the Public/Private scenarios.