What does ‘2 degrees of freedom’ mean in requirement 4b?

Thank you for your question. The 2 degrees of freedom refers to the robot not just the attachment. This is just to make sure the robot has some complexity. 2 degrees of freedom can be forward, backward and turning. If you use two motors you have two degrees of freedom. LEGO is capable of three degrees of freedom because it has three output ports. I hope that is helpful.