The shield has 84 rgb leds. In the technical specs we read that a max current is 2.5 A. Each colour component of one rgb led could draw 20 mA, so theoretically we could run 5 A through 84 rgb leds. I'm sure there's multiplexing going on inside the inner logics and circuits of the shield, so 2.5 A might very well be the highest current the setup ever uses. Even 2.5 A sounds like a huge current. I'm afraid to light up all leds to their brightest values to check if it works. Because it would draw the current from the USB. Through the Arduino.
You can solder an external power source for the shield, but the documents don't mention that any further. What will happen? If the shield is attached to say an Arduino MKR 1010 and the external 5V 2.5A power source is attached to the shield, can the Arduino function without a power source of its own, using the shield power, getting the power from VIN?
It suprises me that the documentation doesn't firmly recommend an external power source. 84 bright rgb leds are hungry!