Generally speaking, we use grade 304 stainless steel with a #4 finish (a brushed consistency) or a mirrored finish (for specific aesthetic applications). It’s the most common grade of stainless, and arguably the most versatile, with a sufficient tolerance and tensile strength for most of our applications. It can be fairly easily manipulated for forming and welding, and is excellent in kitchen and outdoor areas, whereas some other grades are more brittle and not as easily used for fabrication. Also, you’ll never see corrosion near the weld joints with 304 (this is, of course, assuming the welding is done properly).
The grade and gauge are both important considerations when commissioning a stainless steel piece, and your fabricator should be able to recommend something appropriate. However, if you’re not terribly averse to learning about steel composition you can always do some educational reading on the Stainless Steel Corporation website where they have some helpful info.: http://www.stainlesssales.com/stainless-steel-grades.html