Surface saw a long succession of pens, all built on the the N-trig technology. It uses the same digitizer for pen and touch. It needs a battery for to work. These were initially two batteries, one for Bluetooth and another one for the tip. Then Microsoft eliminated the second battery and then pen could work with a single AAAA battery. The Slim Pen replaced this type of battery and became rechargeable, either in the pen garage on the keyboard cover or in a separate inductive charger.
I like the solution with the keyboard cover, because it secures the pen against the screen when closed. This is much easier to handle than any other solution, even better than the ThinkPad stylus which is rather small. The slim pen is flat and has a button at the upper end which doubles as an eraser and a second button on the side.
Slim Pen 2 improves on the design by moving the forward button from the tip of your index finger to your thumb, which feels a bit more natural. The tip of the pen got more pointy and there is a motor inside which provides haptic feedback.
This is a very minor update. It looks different but gives exactly the same feeling when writing. The haptic feedback seems weird to me. It vibrates when you erase something for instance. I toned it all the way down. I like the pointy look but I would not replace a Slim Pen with a Slim Pen 2.
Both pens can be charged in the same places and they pair automatically with the Surface Pro the keyboard cover is attached to.