A little bit of tinkering with creating intentional errors goes a long way towards understanding errors not created on purpose.
Here is an example of a short from power to ground:
The text indicates that Pin 2 is connected to Pin 1 of BT1; this is clearly a short. The short is not at the battery, but was intentionally created at the left of the screen grab. The tricky part is that the arrow is not where the problem actually is on the schematic. The text is however, describing the issue perfectly.
Here is an example of two power outputs being tied together:
The text indicates there is a power flag attached to pin Pin1 of the battery; and there is.
Power flags are pins assigned as "Power output". But any pin can be assigned as a "Power output" if it makes sense. A battery having it's pins set as "Power output" is a good example; same for the output pin on a regulator.
The reason that pwr symbols are not set as "Power output" is so that they can be conveniently used to de-clutter the schematic:
As can be seen from above, the DRC will run just fine as long as there is one, and only one, "Power output" on a net.