The USB section of the System Report actually provides data for each of the USB devices connected to your Mac.
The information you're asking about relates to each of those devices. More specifically:
As far as the maximum power output available, this depends. By default the USB 3 ports of your iMac (the ones you're referring to as USB-A) offer up to 900mA. However, for this to occur you must be using a USB 3 cable. If a USB 2 cable is being used it will limit the available power current to 500mA. And, as mentioned above, the amount of power offered in the case of some connected Apple devices can exceed these figures under the right circumstances.
As for the Thunderbolt 3 (USB-C) ports, these can supply 15W of power per port to external devices. They can even be used for charging MacBooks (although they're not intended for that purpose and it's not something I'd recommend, plus the rate of charge will be quite slow).
Below is some further reading from Apple's website that you may find relevant: