Olympus calls it "fill flash" The symbol is just a plain lighting symbol. I don't want to become too deeply immersed in the debate regarding settings for Inon strobes and the necessity of having the magnet in. However, if you set the Inon strobe to STTL it expects a pre-flash from the camera (which it mimics) to adjust the exposure settings and then mimics the real flash from the camera. If you set it for 1/4 power, then there is no pre-flash and the Inon will mimic the 1/4 power but never sees the real flash and doesn't fire at full power.
The usual recommended approach by Inon is for STTL is to set the camera flash at the "fill" setting which should give proper STTL flash if the strobe is set for STTL. I cannot recall if this camera has STTL in manual mode but I believe it does. If setting to manual flash then set the camera flash 1/4 flash and the strobe should be set to manual mode (you can adjust flash duration on the strobe if you overexpose or underexpose in manual). You are supposed to have the magnet installed because in manual mode there is no pre-flash with this camera. I know that someone on this forum believes that the magnet is not needed in any event. He or she may well be right but the foregoing is what is recommended by Inon. In this mode the Inon flash is supposed to do a full or adjusted flash dump as soon as the camera flash fires at 1/4 power.