The business code you are referring to is the NAICS (North American Industry Classification System) code, a 6-digit business activity code. This code system is used by business and government to classify business establishments according to type of economic activity; in the case of the IRS, it helps the IRS classify the type of business you are operating for purposes of reviewing your return.
In the case of sewing alterations, the appropriate NAICS code is:
811490 Clothing repair shops, alterations only
This assumes that your business involves garment alteration and/or repair, and not retailing new garments, which would fall under a different code.
You can use the following link to enter a keyword for the type of business you are in, and to find the appropriate six-digit NAICS code: