Seems weird to call it a decoder. There's no correlation between VIN and plate other than a plate is assigned to a VIN when it is registered. It's not like the plate number is generated based on the value of the VIN.
"VIN Decoder" is an auto industry term for software that takes a VIN and outputs human-readable information about the car. Historically they printed information encoded into the VIN, and over time database lookups were added. The name is now anachronistic.
Fair enough. I suppose I can see it from the VIN side as "decoding" it.
The plate is matched with a VIN using DMV records, and then we decode the VIN to show Make, Model, Year, etc