If you are not familiar with these, these are known to be two types of UPS. These will both have the ability to supply power to different devices if in case there will be a power outage. The offline UPS will provide the needed power to the load so that the ON transfer switch.
This means that it will get its supply from the battery. The online UPS will supply power not only to the inverter. It will also serve its power to the battery. There will be more heat dissipation for online UPS as compared to offline UPS because offline UPS will only turn on when a power outage already takes place.