This is designed to fit the included water reservoir or the "Simply" brand of juice bottles (and potentially other wide-mouth bottles). It acts as a delayed watering system, it dispenses water below the soil surface directly to the roots of the plants, reducing water loss from evaporation and drainage.
The flow rate is controlled by 5 flow restricted inserts that can be modified and printed separately from of the spike cap to allow me play with/ adjust the flow rate without having to reprint the entire thing each time.
There is also a TPU seal that can be added to help if the cap doesn't seal with the bottle. I've found that as long as print quality is high in that area, the seal is not usually required.
The printable reservoir holds just over 1 liter of water which works well for many indoor or potted plant applications.