I recently bought a laptop that is able to be powered via USB-C Power Delivery 3.0 on 100W (20V). From what I understand, PD (3.0) can be used to charge the battery, but also 'run' a device, i.e. the laptop recognises the power of the PD charger, and performance is reduced in order to cope with the lower wattage via USB-C instead of the default AC adapter. As I perceived it, it thus should not drain the battery on the way, and it should be even possible to run it off an empty battery (battery passthrough), given that it acts as a lower-power AC adapter.
Is that the case, or did I understand the concept of Power Delivery wrong and it only ever charges and discharges devices?