Is US Source Income (for excluding NRAs from plan testing) defined based on the applicable treaty or the general rule in the IRC?

[JDuns]

Where a controlled group employs individuals in multiple countries, should the plan use the general rule set out in the code to determine US source income or may the plan rely on the applicable treaties?

I.e., do you treat non-resident aliens as excludable if (a) the portion of their compensation attributable to services performed in the US exceeds $3000 (the Code answer), (b) if they work in the US more than the number of days specified in the treaty (the answer under most treaties), or © they are not US citizens or you know that are resident aliens (ignoring the US source income portion of the exclusion)?