When opening a bounty and a question doesn't yet have an answer that I'm willing to accept, it always takes me a moment to pick a bounty reason: I am never quite sure what scenarios the "canonical answer" reason applies to.
Canonical answer required
The question is widely applicable to a large audience. A detailed canonical answer is required to address all the concerns.
When there are already one or two answers to the question that cover a bit of ground, it always seems to me like the "Draw attention" and "Improve details" reasons are also applicable. "Improve details" to me suggests that someone should edit the existing answer, though then the person who did the effort won't be the one getting the bounty; so maybe "Improve details" should be understood as "create a new answer based on an existing one / under acknowledgement that one of the existing ones is already a partial answer", but to me this would just increase the mystery of the "Canonical answer required" category.
In which cases should one use the "Canonical answer required" bounty category? If "definitive" or "complete" is meant, one could clarify the bounty description.