I keep seeing "Steam" for software. What exactly is Steam and how does it differ, if at all, from say the boxed version of a program. Such as SPLAT vs SPLAT from Steam?

Its a gaming platform that 'lets' you buy your games online and download and access them all from Steam. Its convenient and they have lots of sales. Also many games from Humble Bundle and Green Man Gaming are really just codes you enter into Steam to get the games. They also have some programs like Cakewalk DAWs, Gamemaker programs and the like and movies.

The difference is in the authorization. Steam allows install and use all software in your account on any computer, so problems like "computer bound" installation and "lost" installation files do not exist when using Steam. Also it handles updates. But that comes with price, the computer should be connected to the Internet (not only for installation but in general to run the program).
I do not have Sonar Steam edition, so I do not know if Internet connection is required to run Platinum. Steam has own "forum", monitored by CW. I think you can get more precise answer there.

Steam can work offline (though how that works and how a specific game/application works can vary by title).

steam edition is like the basic version (home/artist/whateveritscallednow) and you do not need to be connected to the internet, or have steam running (after the initial authorisations), to use it, which is also true of most stuff on steam... steam is just(!) the content delivery/authorisation system...
seth is very responsive on the forum there...

Thanks for the replies. I saw a post earlier about some software and was curious what it was all about.