Oxygen Flow control regulators and conserving devices are pressure reducing devices designed to regulate or lower oxygen pressure from a cylinder to levels that can be safely used by the patient. A Regulator simply regulates the (free) flow from an oxygen cylinder. A Conserving Device is an entirely different kind of device that delivers a "pulse" of oxygen as the patient inhales (on demand).
Regulators and Conserving Devices are selected based on the type of valve connection to your oxygen cylinder. Most use either a CGA870 or CGA540 valve for connecting to the cylinder. Oxygen output (to cannula) is via a BARB or DISS outlet. Barb is just like the name states - it's a barb type connector that your tubing is forced onto. The DISS type connector is a screw-on connector.