Under what conditions is an Always Free Compute instance terminated for being unused?

The Oracle Cloud Infrastructure Documentation says:

"Free accounts remain active and available to you as long as the account has been used within the past 60 days."

But it does not specify what counts as "used". I'm using my Always Free Compute instance to host a public server. What is the exact condition for usage here? Logging into the account? The instance receiving any external network traffic? SSH or FTP access?

I would rather not log into my account periodically just to keep it alive. I want the server to continue running and being accessible passively. But I don't know what counts as active usage here, so I don't know if that's necessary. I can't find any documentation on this.