Great answer, but allow me to clarify a bit more. The kayfun 3.1 has a "collector" on the base section. It's there to collect juices which might overflow, although mine has really never collected anything. This makes the base about 1/2 inch tall, of which the bottom 1/8 inch or (with the 510 connector) unscrews so that you can clean out the tank. The 3.1 also has three "holes" around the perimeter of the base, one has a screw for airflow adjustment, the middle one is the air hole, and the one on the right is the fill valve. I have 2 Kayfun 3.1s, one has a screw in the fill hole allowing you to fill with a needle or needle tip bottle, and the other has a "one way valve" fill. It's a spring loaded ball bearing which you push on with the needle/needle tip bottle to fill, and when you remove it, it should close (doesn't always work very well. The 3.1s have a screw in drip tip which needs an adapter to use a regular drip tip. (one of mine has the adapter, one doesn't).
The Kayfun Lite - on the Lite they removed the "collector tank" since it didn't do much anyway. Now the base is only about 1/8 inch tall (on the outside ring). The outer ring on the base has only one hole, for airflow. Since I never had any issues with airflow, the missing airflow adjustment was never an issue with me.
The fill hole is a simple screw in a hole now located on the bottom, and you still need to use a needle or needle bottle to fill it, and the drip tip is now a regular 510 drip tip mount (some fit, some don't).
The Kayfun Lite Plus is very much like the "lite", but now has two screws on the bottom instead of the one fill hole. The have re-established the adjustable airflow and moved it to the bottom, and the fill hole is now wider and deeper, like a little funnel. This allows you to fill it with a regular size eliquid bottle. The screw for the fill hole now has a small O ring to prevent it from leaking. It still has the 510 type drip tip.
Between the 3 models it appears that they have been making slight improvements along the way. From the 3.1 being big and heavy and hard to fill and using a proprietary drip tip, to the new Lite Plus which is much easier to fill, has adjustable airflow, is lighter, and uses standard drip tips. On the inside all 3 models are about the same, same build deck, chimney, etc.
I like the improvement that they have done, but also I've vaped many a tank through all three models, and they are all very good RBAs. Any one that you can get your hands on will be good, but I very much like all the little improvements added with the Lite Plus.
Hope this helps!
