This help article was written with regards to the initial release of OmniFocus 3 in OmniFocus 3. This database format is not compatible with OmniFocus 2. Do not migrate your database if you need to sync with OmniFocus 2. In summary:
The OmniFocus web add-on subscriptions exist for customers who already own a standalone app, but wish to add web access. The web add-on subscription is only available through the Omni Store. Subscriptions are managed through your Omni account. Video OmniFocus; What’s in Pro? Explore the three features that define OmniFocus Pro: Focus, Custom Perspectives, and Scripting. As of version (Released July 26, ) OmniFocus 2 for iOS is a free download in the App Store, with the Standard and Pro feature sets available via In-App Purchase. We also offer a day free trial, after which OmniFocus will function as a free viewer. The purchase options available to you are based upon your purchase history.
Creating Projects and Groups with OmniFocus 2 for Mac