When the Boron is powered via USB, and Vusb is used as an output to power a peripheral, the Vusb port will output 4.5-5v and the output current is limited to the USB port specification. USB v2.0 and prior ports are limited to 500mA. USB 3.0 and newer may supply more current but are not required to. So, if you account for the power draw of the Particle device, you can pull about 450mA from the USB port.
When Vusb is used to power the Particle Device, then the voltage is limited to less than 12V. There’s no limit to the current, the Particle Device will pull what it needs. You have to make sure your external power supply will meet the current demands of the Particle Device and whatever peripherals are also powered by the external supply.
I added a pull request to update the Boron docs to add the “input” scenario.