Twenty-two mil veneer is a sheet of raw veneer (called a 'face' in the trade) that has been backed with two layers of paper backer before it is flexed and sanded.  A well-made piece of twenty-two mil veneer will have a layer of melamine/resin acrylic, or some other appropriate substance between each layer of wood, paper, and the the second sheet of paper.

The purpose of this is to bond the paper to paper and to the wood veneer itself. This also creates an impenetrable moisture barrier between the adhesive and the wood veneer stain or finish. This moisture barrier competently prevents both bubbling and cracking of veneered projects. If you've ever had this problem, you know just how valuable this can be.

While not quite as flexible as the 10-mil single-ply veneer, 22-mil paperback, such as Oakwood's Bubble-Free veneer, still enjoys an impressive flexibility for most veneer applications. It bends well around corners and curves, allowing for interesting and exciting designs.

Its substantial thickness covers most substrate imperfections, a definite plus when working on projects in the real world. Twenty-two mil veneer exhibits no black seam line such as those seen in other non-paper backed veneers, and it is sandable. Keep in mind that the only difference between 22-mil veneer and 10-mil single-ply veneer is the thickness of the paper backer. 

The wood veneer itself is the same thickness no matter the backer. So treat both veneer types with the same care when sanding. Altogether, 22-mil veneer is an attractive and usable product well worth its moderately increased cost over the basic 10-mil wood veneer.