What is the small knob on top of tranny case for?

The small knob on top of a transmission case is typically a fill plug or breather. The fill plug allows for adding Transmission Fluid to the system, while the breather helps maintain proper pressure and prevents fluid from leaking out due to temperature changes. It's important for ensuring that the transmission operates efficiently and remains properly lubricated. Always refer to the vehicle's manual for specific information regarding its function.