Tabs are usually constructed of heavy stock, to withstand repeated use. There are many choices available, ranging from the most basic 36# white or manila, up to exotic or extra-heavyweight stocks.
In linens, you look for thread count, and in stationery, you look for paper weight and fiber content. For running off documents in your office, 20 to 40 lb. paper is standard. However, fine stationery requires a heavy feel and is usually printed on 80 to 100 lb. stock. In addition, papers come in a variety of fibers, most typically cotton, linen or wood based pulp. Cotton and linen papers tend to be the finest and most expensive. Combinations can also be quite appealing at a slightly lower cost.