A customer needed to read some simple information out of our server software programmatically, and suggested that a "SOAP API" might be a good way to do that. To me fell the task of researching what that might mean.
SOAP is a lightweight protocol intended for exchanging structured information in a decentralized, distributed environment. Why, it says so right in the spec!
I dread to think of a protocol that the W3C would call "heavyweight".