What does the 16“ in ”16-Inch Full Ext Drawer” refer to

In the U.S. the item you refer to is most often called a draw catch. There are other names, depending on manufacturer and purpose, but I worked 19 years in commercial and residential hardware supply and we always called that a draw catch.

It does not have a consistent meaning, but in the context of building materials/products is usually intended to suggest a higher level of quality or prestige. I think this comes from the suggestion that this particular material might be specified by an architect, who wants a quality project and doesn't care much about price. In contrast, builder-grade products are low-quality, low-price, generally acceptable items available at low cost for builders who want to limit expenses.

In some areas a more specific meaning has evolved - the shingles you point out are an example, where architectural shingles are a class with a typical design (in contrast to cheaper 3-tab shingles).