What does the "high gain" option refer to when buying preamp tubes

I received an email from tube depot and I was browsing 6SN7s and noted that one of the options available was "high gain."

Does this mean that selecting that option for a tube would increase gain, or does it mean that selecting that option would be a better tube for preamps/systems that are already high gain?

Or am I cold on both guesses?

Thanks in advance/Matt