DOT codes are US government tyre identification numbers. Each DOT code will usually comprise of 11 or 12 letters and numbers and gives information about, amongst other things, where and when the tyre was manufactured. The first two letters indicate the plant in which the tyre was produced and the last four indicate when it was made.
For example, a DOT code beginning WA and ending 2102 refers to a tyre built in the Firestone factory in Bethune, France in week 21 of the year 2002. Each manufacturer will have several factory codes referring to various plants around the world.
Any other numbers or letters contained within the DOT code are used by the tyre manufacturers to identify tyre size, construction and pattern etc.