Where I live 0.058 is standard on Stihl saws with .325" cutting attachment, while 0.063 is standard with 3/8". 0.050 is only used for Picco Micro.
I must admit that I don't like that situation at all! I sure have a hard time getting 0.050 3/8 RSC over here, and think that I have to settle for Oregon 72 LP.
In addition, at least on some markets, it seems that Stihl want to sell chain to people with other brands of saws and/or cutting attachment.
Not so in Norway, though. :angry2:
The gauge of a chain is the width in inches of the drive link, which must match the groove width of the guide bar. The standard widths for hand-held saws are .043, .050, .058, and .063. The .058 gauge chains are more popular in Europe and Canada, and .050 is the most common gauge in the U.S.
Husky saws might be the primary target as they are the most numerous non-Stihl saws, and often sold with .058 bars, but there are lots of other saws also, that these chains will fit.
It isn't really a question of saw brand, but the gauge of the bar somebody has put on it.