What does the digit mean?
just means how old the steam account is. theres 1-8 digits ( maybe 9 now, but i havent played for several months ), 1-2 digit are basically impossible to find because they were most likely beta testers for steam... 3 digits are basically just as hard, but not testers, 4 digits are somewhat hard to find, not many people sell them. 5 and 6 digits are the most popular steamid's sold, and arent too hard to find, but not too common either. 7 digits are like 3 years old ( i think? ) and any account you make now are going to be 8 digits. its just an identification number.
people usually buy lower digits because they think it makes them good or something when people look at it through the console in counter-strike. lots of people think if your account is old then you must be good or something, people are dumbasses. people who buy 4-6 digits usually suck dick at the game, and if you can find a 1-3 digit somebody will sell SOMEHOW, i personally feel it would be worth it to buy, but good luck cuz its extremely rare and it'll cost u about $2,000 for one. ( my guess, possibly more, i dont think it will sell for less )
theres A LOT more to steamids i can tell you, especially once you get into competitive leagues like twl ( lol shitties - normal people ), ogl ( better, but still shitties lol - normal people ), cal ( good league, i got to the highest division in cs:s, invite, and high up in cs:cz and 1.6 - normal people ), cevo ( extremely good league, you can earn money by winning - normal people ), cpl ( extremely high pay, ex: $25,000 for tournaments held around the world - these are actual professional players. just like basketball teams, baseball, soccer, anything... but for counter-strike ), and some more leagues im sure i missed. but thats more than a good enough explanation for you above lol.
