If I buy a USB-C PD power bank with 10,000mAh capacity, per the USB spec it can output power at 5V, 9V, 15V, or 20V, depending on which device is plugged in. W

If I buy a USB-C PD power bank with 10,000mAh capacity, per the USB spec it can output power at 5V, 9V, 15V, or 20V, depending on which device is plugged in. Which voltage is the 10,000mAh capacity in reference to? Most likely it's actually the internal 3.7v cell voltage, which is even less relevant than any of the potential output voltages. Your argument makes sense for purchasing raw battery cells (li-ion or plain old AAs) but as soon as those cells are integrated into a sealed consumer electronics device (battery pack, smartphone, laptop) it is no longer an interchangeable unit with a known voltage.