How does USB-C affect USB's distinction between "host" and "peripheral"?

Solution 1:

Since this wasn't closed I will answer it here.

In USB C the roles a device can play is separate data and power wise. These are all negotiated by sending data packets over the configuration channel - USB C has a separate CC wire for this which is missing from legacy A/B connectors. This is all prescribed in the USB Power Delivery standard. (While the USB Type-C standard uses the USB PD protocol, USB PD could, in theory, be used elsewhere.)

Power roles can be

1. provider also called source
2. consumer also called sink.
3. dual role power (DRP) port.

For data

1. No data
2. downstream facing port (DFP) -- this means it sends the data.
3. upstream facing port (UFP) this means the device receives the data. A monitor or a mouse is an example.
4. dual role data port (DRD). The port’s power role at attach determines its initial role. A source port takes on the data role of a DFP, while the sink port takes on the data role of a UFP. If a port is both DRP and DRD connecting them together will randomly lock one to source and DFP and the other to sink and UFP. Once this initial connection is done, role swaps can be negotiated as needed.

As we know, USB C is very flexible (in less charitable terms: it's a gigantic mess) and it's very hard to give good examples in the list above. For example, USB C ports on a portable monitor might be:

1. A power sink with no data roles -- this might serve the purpose of powering the monitor from a USB C charger while the video is provided over say HDMI or even from a laptop on another USB C port which can't provide adequate wattage.
2. An UFP for video and potentially data. ​This will likely be a power sink.
3. If it has a built in battery it might be an UFP and a power source to charge the device it gets the video from. It is also possible there's a menu option to switch between source and sink roles so either a laptop can charge the monitor or the monitor can charge the laptop.
4. Again, if there is a battery inside, it might serve as a docking station too so it can provide a port which is DFP and power source.
5. Instead of the battery inside the monitor, it might ship with a battery bank crossed with a docking station and for that it might provide a DFP which is a power sink.

As for the example in the question, if you have an adapter which provides a USB A receptacle and it is up to standard it will sport a USB C male connector on the other end. Plugging the USB C end into a device will force the device to assume a DFP role and it will also provide 5V to the legacy USB device connected. This is neither source or sink, it's a legacy mode. Instead of nice negotiation, it's forced via some resistors. BTW there's no such thing as "USB C OTG", that's marketing term.