If you have accounts receivables from a U.S. entity, you may get the request for a W8-BEN or a W8-Ben-E form. Without this form, the U.S. entity that pays your accounts receivable will withhold a non-resident tax. The only way to get the refund of the tax is to file a U.S. non-resident return, either a 1040NR for individuals or a 1120F for corporations. The waiver form indicates to the payor that you are exempt from U.S. taxation under the Canada/U.S. tax treaty.
Note that if you are considered to be carrying on business in the United States, by virtue of selling or marketing your products in the United States or providing services in the United States (such as engineers, consultants, etc.),then a waiver may not apply or be available to you if it is considered that you also have a permanent establishment in the United States by virtue of Article V of the treaty. In this regard you should obtain professional advice on you U.S. tax and compliance obligations. Generally a protective tax return is filed to ensure you avail yourself of available deductions against US source gross income should the IRS take a contrary view.