Is international tax law part of international law? To an international lawyer, the question posed probably seems ridiculous. Of course international tax law is part of international law, just like tax treaties are treaties. But to an international tax lawyer, the question probably seems less obvious, because most international tax lawyers do not think of themselves primarily as international lawyers (public or private), but rather as tax lawyers who happen to deal with crossborder transactions. And indeed, once one delves into the details, it becomes clear that in some ways international tax law is different from "regular" international law. For example, international tax lawyers talk about residence and source jurisdiction, not nationality and territoriality, and the different names also carry different content. And while tax treaties are indeed treaties, they are concluded differently than other treaties (for example, they are negotiated by Treasury, not the State Department), are subject to different modes of interpretation (the Vienna Convention on the Law of Treaties (VCLT), the "bible" of the international lawyer, is rarely invoked), and in the United States are subject to a rather peculiar mode of unilateral change, the treaty override.
Avi-Yonah, Reuven S. "International Tax as International Law." Tax L. Rev. 57, no. 4 (2004): 483-501.