another better explaination may be for RSN's that carry 2 MLB teams, like ComCast Sports Net Chicago. Many times during the year, the Cubs and White Sox are playing at the same time with CSN carrying both games, one game is on channel 429, the other on the sports alternate. Same with FSN Southwest, Sun Sports, and FSN North.
If you live in an area that I described above, you'll be able to tune into a game on one of the "sports alternate channels" almost every night, if not, more than likely every game on the alternate channels will be blacked out for you. Where I live, the Milwaukee Brewers are on the alternate channels and if I had the KC Royals, which I don't, even though i'm less than 3 hours from KC, which is a topic for another day, the Royals telecasts would fall on the sports altnerate channels as well.