The PCT relates mostly to claims for priority of patents, so if a document is files in say the US, the applicant may then later (within a specified time period)

The PCT relates mostly to claims for priority of patents, so if a document is files in say the US, the applicant may then later (within a specified time period) file an application in another PCT country and claim the same priority date as the US application (ie, the date of filing). All patents are applicable within a single nation only, so just because company X has patent Y in country Z down not mean you can't use the invention in country W unless X has a patent to the same invention in country W as well. However, if they've filed using the PCT they will by default designate they wish for all member countries to examine the patent, so until that patent has lapsed in each country, there is a possibility that the inventor may persue protection and you may then be infringing that patent. Fun stuff.