Integration over time and integration over bit fields are both ways we do deep sky astrophotography.
Integration over time is used to collect more photons without saturating sensor wells and integrating bit fields (stacking) is used to increase signal to noise
What does that mean exactly? What's the relation between integration time and exposure time?
I think this is their terminology for exposure time. The sensor is integrating charge from incident photons during this period. Of course the image could be stacked also!
Edit: the sensor is integrating CURRENT. Charge is the integral!