这种税制只发生在联邦体制的国家，中国的体制非联邦体制，要找到相应的译词简直不可能。

http://www.infoplease.com/ipa/A0005948.html
这个链接里说的是在美国各州应该分派的所得税：
A state is empowered to tax any net corporate income attributable to in-state activity no matter where the corporation is headquartered. Corporations must apportion their income among the states where they have a sufficient connection (the legal term is nexus) and pay tax in each of those states. Several states tax unincorporated businesses separately.

这也就是说，跨州营运的公司必须按照 (美国税制订定的) 规则，将他们应缴的总体所得税分派缴纳到与其营运有关系的各州 (法律上称 nexus)。

由于所得税是“直接税”的一种，因此我尝试将 nexus tax 翻译成“直接关系税”。

你的句子里问的是 "Where do you have nexus for sales/use tax purpose?"，可见这个税也包含销售时需缴纳的 VAT 或一般租税等直接税，因此翻译成“直接关系税”更不离谱。问题只在于文本里对此是否有明确的定义。比方说下面的链接，是一份加盟事业的合同模板，Article 3 所做的定义就包含了 nexus tax。

http://big5.lawyee.com/Contract/Contract_Display_EN.asp?Chan...

NEXUS TAX means any Tax imposed by way of withholding in respect of or in lieu of an Excluded Tax, but only to the extent such Tax would not have been imposed but for the nexus (other than as a consequence of the activities of the Contractor) of the Purchaser or its affiliate to the jurisdiction imposing the Tax.


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Note added at 1 hr (2007-07-21 02:41:06 GMT)
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http://www.allbusiness.com/accounting-reporting/corporate-ta...
这个链接说到的 nexus 指的也是同样的税制，其中说到的 apportionment 当然也包含 nexus tax 的征收规则：
This two-part article discusses the most significant recent state tax developments in the corporate income tax area. Part II describes some of the more important developments in apportionment formulas, filing methods/unitary groups, administration, tax increases and other significant miscellaneous issues.

During 2003, numerous state statutes were added, deleted or modified; court cases were decided; regulations were proposed, issued and modified; and bulletins and rulings were issued, released and withdrawn. Part 1 of this article, in the March 2004 issue, focused on nexus, tax base, business/nonbusiness income and trademark/tradename issues. Part II, below, covers some of the more important developments in apportionment formulas, unitary groups/filing methods, administration and other significant corporate state tax issues.


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Note added at 7 hrs (2007-07-21 09:12:31 GMT)
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基本上，nexus 的意思就是 Lu Zou 说的“关系/关连”。

至于所问的语句，我则同意 Shenzhen Borlan的说法：
“在何处您因销售/使用而可视为(对该州)有税务关系？”
“在何处您因销售/使用而产生(对该州)的税务关系？”

但若问的是 nexus tax，我还是会用“直接关系税”。


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Note added at 7 hrs (2007-07-21 09:28:02 GMT)
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在 Shenzhen Borlan 的 Example sentence 里很清楚说明 nexus 是什么东西：“如果州外的事业与田纳西州没有关系，该事业无须缴纳田纳西的税捐。(If the out-of-state business does not have nexus in Tennessee, the business is not required to collect Tennessee taxes.)”

在中国的税制里会是怎样的情形呢？公司设在河南郑州，货物却存放在其它各省的仓库里，销售订单都往郑州去，发货却由各省仓库派出，那么该公司只在郑州缴税吗？或是由于那些仓库也必须在其它各省缴税？如果是后者的情况，那就是所谓的“被视为有 nexus”了。因此称为“税务关系”应该是比较可以接受的。