Hello professor, I hope you are doing well !

I have a question about lesson 12 especially activity 4 types of interaction.

Some specific part is kind hard to understand for me. So i have question.
I've learned of different types of interaction.
In here, "S > S is demonstration from one student to another, pair work."
And "Ss > Ss is pair work or group work."

What is the exact difference of two of them. Actually not about difference, but how could i discriminate of them?

Thank you.