The alternating current (AC) capacity is that of the inverter, and is the max instantaneous power output.
AC capacity factor is significantly determined by plant design. You can nearly double an AC capacity factor by installing more modules - but that's expensive!
From DC capacity, you can estimate land use, capex, and with publically available climate data, energy output.
2. If only recording one PV capacity, DC is much better.
3. Grid operators in the US, Thailand and Japan publish mostly AC capacity. Which is fair, because they are concerned with wire size. Nearly everyone else uses DC.
CFs above this are likely to be either AC, or using tracker devices, or both. (Trackers are a topic for another day!).