What does ring and cylinder symbol mean?
It looks like a current transformer. It would make a lot of sense if there was a current sensor of some type in the circuit. It monitors the current swing in the boost inductor, to switch off the low side FET when the inductor current exceeds some threshhold.
Whether it's a real current transformer (I'm not sure how they could integrate the magnetics in an IC) or an active device that behaves like one is another matter. There's no description of it in the data sheet. Hearth in comments suggests it could be a Hall sensor, which I think is probably more likely than a flux coupled transformer.