Generally speaking, a lot of firms follow best practice where "issues"/"tasks"/"stories" are written (by a Product Manager, or PM) to include both a prescriptive request as well as a list of acceptance criteria that can be checked off to act as a mutually understood list of requirements for that issue to be considered complete.

The "given/when/then" model is basically, "given this circumstance, when the following occurs, the following behavior is observed" as a framing device for building ACs, though not everyone uses that.