Art. 9 CISG
(1) The parties are bound by any usage to which they have agreed and by any practices which they have established between themselves.
(2) The parties are considered, unless otherwise agreed, to have impliedly made applicable to their contract or its formation a usage of which the parties knew or ought to have known and which in international trade is widely known to, and regularly observed by, parties to contracts of the type involved in the particular trade concerned.
The Secretariat Commentary on Art. 8 of the 1978 New York Draft, predecessor to Art. 9 CISG, gives insight into the drafters' unterstanding on the eve of the 1980 Vienna Diplomatic Conference.
Overview of case law on Art. 9 CISG: UNCITRAL Digest
The UNCITRAL Digest on the CISG can be used as a starting point to discover the relevant case law on this article.