There is an area of customary international law that deals with the proper exercise of extraterritorial jurisdiction. Encyclopedia entry here (paywalled) https://opil.ouplaw.com/view/10.1093/law:epil/9780199231690/... Someone seems to have mirrored the first page of the entry here https://ilcfhuns.wixsite.com/ilcfhuns/single-post/2018/03/07...