If you don't know why then there's no use. Basically old accounts are categorized under digits if you might've heard 5 digits, 6 digits, 7 digits and so on. All the steam accounts made on 13th September 2003 (day steam was launched for general public) are 5 digit accounts, and of course rare Accounts made on 16th september till 31st december are 6 digits. All the accounts made later on are 7 digits. If the account had Half Life 1 package you'll get a 5 year veteran coin in CS GO that is much sought out for and pretty nice to have one.
PS - I'm not sure about dates of 6 digits but 5 digits dates are correct.
PPS - I've sold a 3 digit account for 500$ myself and a 4 digit for 200$.
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