VIN when powered over USB question

The documentation states

When the Photon is powered via the USB port,VIN will output a voltage of approximately 4.8VDC… the max load on VIN is 1A.

Does this mean I have an entire ~4.8W to play with? Or do I need to take in to account the power the Photon will use as well?

And how exactly does this work when the device is powered from a PC’s USB connection (as opposed to a high current USB charger)? Its been a while since I read the USB specification, but if I recall correctly, USB1/2 devices are limited to 100mA when they first connect, and can negotiate with the host to consume up to 500mA. Does the Photon honor the specification, and regulate the current output to prevent a overload? If not, is there any way to determine if the Photon is connected to a PC and not a high current USB charger?

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