An Oxygen Transfilling Concentrator is a machine that separates room air into oxygen and nitrogen. The nitrogen is discarded, while the oxygen is stored, concentrated and delivered to you or your cylinder in a controlled dose at 91% to 95% purity.
Your unit may or may not be a single unit attached to the stationary concentrator.
A dual purpose concentrator will allow you to receive oxygen at home while you fill tanks at the same time. This will allow you to always have the portability you need at your convenience.