Adding a tad more (as it pertains to OUR industry)...
Sodium Metasilicate used to be a oft used detergent builder when a higher alkalinity is preferred and because of a secondary characteristic of the chemical. It also acts counter-intuitively as a pretty darn good inexpensive corrosion inhibitor. Rarely used today, because of the alkalinity being quite a bit too high for 5th generation carpet fibers. It also tends to leave the carpet with a harsh hand (stiff), if left as any residue. Sodium Metasilicate acts by plating out on reactive metals, such as brass. The downside is that the plating out is cumulative, causing heat conduction problems in heat exchangers, and fouling of small orifices that require periodic descaling to remove. Much better corrosion inhibitors are generally used today that don't contribute to hardware scaling and don't effect the fibers being cleaned negatively. As well, progress has been made to clean with lower pH's.
Sodium Metabisulfite is a medium-strong reducing agent, very popularly used in tannin and coffee stain removers. It's strong points are that it is relatively inexpensive, and generally safe to most dye systems used in carpet. Some have a dislike for it's strung, pungent odor which is also quite irritating if you have to use more than a little on a particular project. Some coffee stains don't respond well to reducing agents, as many newer "designer" and nearly all decaf coffees now use artificial colors that respond better to oxidizer type stain removers.
This post is certified as 100% cut-n-paste free. LOL