[rjh29]

It's easier to split USB 3 into two stages:

1) USB 2.0 - 480 Mbps, 0.5A (2.5W / 5V)

2) USB 3.0 on Type-A - increases data to 5/10 Gbps (e.g. for SSDs) and power to 0.9A (4.5W / 5V).

3) USB 3.0 on Type-C - adds pins to enable most of the new stuff: base 3A (15W / 5V) and up to 240W with power delivery; extra data lanes for 20 Gbps, DisplayPort for external monitors.

4) USB 4.0 on Type-C - changes protocol completely (using packets instead of fixed lanes per pin) and requires minimum speeds and the use of type-C port.

A-to-C cables act like A-to-A as they don't have extra pins - they're just easier to use.

Pre-Power Delivery lots of phone chargers used A-to-C cables with a custom protocol to negotiate power draw beyond 0.9A. And some devices pull 3A anyway just assuming there's a power brick that supports 3A attached. But technically none of that is compliant.