Unfortunately there isn't any consistency across different type foundries in using “Pro” or “Std” names, and what to expect from fonts using these names. A “Pro” font from one foundry, might support compeletly different languages and contain a different amount of OpenType features than a “Pro” font from another foundry. You need to read the documentation of every foundry (or every typeface) to understand what the “Pro” and “Std” extensions mean.
Since the complete Underware font library has been upgraded in April 2013, all Underware fonts are named Pro. They are named Pro because they have a large language support (200+ languages), contain multiple OpenType-features, better cross-platform compatibility and have a much better screen performance. Next to that, it’s handy to quickly distinguish these new fonts from their previous, inferior versions.