Article 10 CISG
For the purposes of this Convention:
(a) if a party has more than one place of business, the place of business is that which has the closest relationship to the contract and its performance, having regard to the circumstances known to or contemplated by the parties at any time before or at the conclusion of the contract;
(b) if a party does not have a place of business, reference is to be made to his habitual residence.