Usb 3 devices can run on up to 900 mA whereas usb 2 ones up to 500 mA. Given that the power pins for usb 3 are separate pins from those for usb 2, does this mean that a usb 3 hub that runs on power from a usb 3 port can send a total of 900 mA to usb 2 devices?

Or is it a total of just 500 mA to usb 2 devices because it comes from the usb 2 pins of the usb 3 port powering the hub?

Or is it a total of 500+900=1400 mA to usb 2 devices?

Or does it depend on the hub manufacturer?

An example of a usb 3 hub that delivers all the 900 mA (or even 1400 mA) to usb 2 devices, each device allowed up to 500 mA?

Are you asking about an external or internal hub?

As far as I know 3.x power pins are the same as 2.0.

Per the spec a 2.0 high powered device can only draw a max of 500ma regardless of how much the port can source. A 3.x high powered device can only draw a max of 900ma regardless of how much the port can source.

Charging ports are different.

The power pins are the same, as michaelk says. This is part of what ensures backward compatibility from USB 3.1 all the way back to USB 1.1 for Type A connectors. There is also a pair of data pins for compatibility, so a total of 4 pins are required to keep everything working together happily. The voltage remains the same for the power pins (5V), but the source current limits increased for USB 3.0. The older devices should not draw more current than their rating, or more than they need.

An exception: Some power hungry devices, such as an external DVD burner that I have (USB 2.0), may need more current. My drive came with a Y-cable that allows it to plug into two USB ports to get the extra power, but I have never had to use it.

Can any usb 3 hub deliver all the 900 mA that it receives to usb 2 devices, for example 500 mA to one device and 400 mA to another? Or 500 to one that needs this much and nothing left for the other, as would happen with a usb 2 hub?

Not as far as know. All the system knows is if the device is low or high power and the total in number of unit loads but not how much current is actually sourced.

What is the design reason for the concept of unit loads and not just talking in terms of mA?

A unit load is a single low power device. I don't know and I am not a USB expert but I assume the specification was a compromise with hardware and software design complexity. USB enumeration is a complex thing and many devices ignore it and don't limit their current. The host power supply design may just meet the min requirements and so can lead to problems.

When a device is connected via enumeration it tells the host if it is a high power device so the the OS knows how many units are connected to a single port. As far as I know there isn't anything that keeps track of current sourced.