Pressure tanks used with under sink reverse osmosis units are miniature versions of pressure tanks used on residential water wells.
Water is stored in a bladder, made of a material called butyl. When there is a demand for water, the water is pushed out of the tank’s bladder by air pressure.
In the picture, the stem at the top of the tank is where water both enters and leaves the tank. The blue cap on the front near the bottom covers a standard Schroeder air valve, like the valve on a bicycle or automobile tire. This is where air is pumped into the tank. The air valve on some tanks is in a different place, so you may have to look for it. In older Amtrol tanks, the air valve is located inconveniently on the bottom of the tank.
When the RO tank is empty of water, it should have a pressure of five to 10 psi air. As water is forced into the tank by operation of the RO unit, pressure inside the tank increases. Standard RO units have a hydraulic shutoff valve that stops production of water when the pressure in the tank reaches approximately 2/3 of the pressure of the tap water that supplies the RO unit. Thus, if your city water pressure is 60 psi, the RO unit will fill the storage tank to around 40 psi before shutoff. If, however, your city pressure is only 50 psi, the RO unit will stop filling the tank when the pressure in the tank reaches a bit over 30 psi. This may make it difficult to supply an icemaker with your RO unit.
Here is a stylized view of what it looks like on the inside of an RO tank. Actual interior construction of tanks may differ, but the illustration shows the basic arrangement.