Many people believe that line-interactive UPS systems are "online" designs, probably confused by the design's marketing name. Actually, the line interactive design is merely an upgrade of the standby UPS and it is an offline type of system.
By adding a voltage autotransformer which can regulate the output voltage, the UPS gains the ability to regulate its output voltage. The inverter of the said UPS system operates at all times, charging the batteries and power the load while the utility power is normal and draws power from the batteries to power the load once the utility power fails.
Because of the autotransformer, the UPS always regulates its output voltage and will not switch to batteries during small undervoltage / overvoltage spikes. This makes it an ideal design for home and small business computers which are powered by an unstable utility power grid. Finally, the presence of the ever-working inverter makes the switching between utility and battery power much smoother, faster and considerably reduced switching transients.