I'm confused about the precise terminology to use when referring to various subjects which are all related to making good choices. I know that axiology is general study of value, including moral, aesthetic, and potentially other types of value. Ethics is the strictly moral subfield of axiology. Within ethics, normative ethics focuses on what constitutes right and wrong behavior. It seems to me that normative ethics can be further subdivided, as it contains separate concerns about what we should morally value, and how to act given a set of moral values. I'll use the example of a utility-maximizing agent to illustrate this distinction, but only hesitantly, because I think it is also relevant to non-utilitarian agents.
The question "what is my utility function?" is obviously important to a utilitarian, who cares whether they will be maximizing aggregate felicity, preference satisfaction, or the share price of Facebook. Equally important, however, is the question "how should I act, in light of my utility function?" The difficulty of this problem may be less obvious, but thought experiments like Newcomb's problem show that two agents with the same answer to the first question can behave differently if they have different answers to the second.
I'd like to say that the second question is asking about normative decision theory, which studies optimal decision-making to maximize value, while taking no stance on what is actually valuable. But this creates a contradiction in my system of definitions: if normative decision theory doesn't concern actual value, then it can't be axiology. Yet normative decision theory still clearly recommends particular behaviors, so it is part of normative ethics. The problem is that ethics is supposedly a subfield of axiology, and we just decided that axiology excludes normative decision theory!
There are multiple resolutions to this contradiction: one could argue that
axiology does in fact encompass normative decision theory,
not all of normative ethics is axiology, or
normative decision theory isn't normative ethics.
There might also be other ways to draw a coherent Venn diagram. I'd be curious to hear any justification for one of these fixes over the others.