The PCT International Search is a high quality search of the relevant patent documents and other technical literature. This search is conducted by an International Search Authority (ISA). Also, it is a mandatory step in the PCT filing procedure. The PCT has prescribed certain standards for the quality of the search. After the ISA completes its search, it issues an international search report to the applicant. It also hands out a written opinion of the international search authority (WOSA). Basically, this determines the patentability of your application.
You must also specify an ISA while filing your application. As of now, there are 15 competent Search Authorities. Therefore, you can select 1 (in some cases more) ISA and it will conduct the search for you.
Also, do make sure that you check out this free e-book: A Quick Guide to Patents. It deals with basics of patent procedures.
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