API Guidance does not explain the term workload. Although, it refers to that term several times - https://github.com/search?q=repo%3Amicrosoft%2Fapi-guidelines%20workload&type=code

There is a need to explain the context of this term using backing service, domain, implementation, deployable runtime, etc.
Given the repo is public, I'm assuming the target audience of this literature is anyone on internet.