*Digit is the length of the Steam ID.
The lower it is, the more unique it is.
A 5 digit would for example be STEAM_0:1:23456. After the last colon, you count the numbers, and then you know what digit it is.
Also, all the 1, 2, 3, 4, 5, 6 digits and low 7 digits are made in 2003, which was the year Steam was made.
September 10th - 1 and 2 digits
September 11th - 3 and low 4 digits
September 12th - High 4 digits (Above 6xxx) and 5 digits.
September 13th - High 5 digits and 6 digits
September 13th to October 21st - 6 digits
October 21st to February 18th - Email Login 7 digits (22xxxxx and below)
February 18th to Late 2005 - Username 7 digits
Late 2005 to now - 8 digits
STEAM_0:0:500000 - Sep 2003
STEAM_0:0:1000000 - Oct 2003
STEAM_0:0:5000000 - Nov 2004
STEAM_0:0:10000000 - Jan 2006
STEAM_0:0:15000000 - Jun 2007
STEAM_0:0:20000000 - Jul 2008
STEAM_0:0:25000000 - May 2009
STEAM_0:0:30000000 - Jan 2010
STEAM_0:0:35000000 - Sep 2010
STEAM_0:0:40000000 - Apr 2011
STEAM_0:0:45000000 - Oct 2011
STEAM_0:0:50000000 - March 2012
STEAM_0:0:55000000 - August 2012