What's the difference between Sol, Terra, and Luna?

Sol is the most powerful for the most complex tasks, Terra is balanced for business, and Luna is faster and less expensive.

Why was the model released in a restricted version?

At the request of the US government for a security review before public access, as a temporary measure.

When will the model be available to the public?

OpenAI stated that a wider release will begin in the coming weeks after the trial period ends.

The US Restricts the Release of GPT-5.6 by OpenAI to Certain Partners | Wabl

OpenAI launches the new GPT-5.6 model (Sol, Terra, and Luna) in a restricted beta version exclusive to US government-approved partners, in a rare preemptive censorship move.

Wabl