"Client applications with the RLZ library can use explicit cohort tagging to manage promotion analysis. A client application with a particular tag can transmit that tag as it chooses for payments and analysis purposes. As an example, the RLZ parameter "rlz=1T4ABCD_enUS202" indicates the client application is Toolbar version 4, distributed with Abcd software bundle, English version, to a US user in December 2006. This empowers computation of metrics broken down into useful dimensions," explains Google.
"T4" is a codename for Google Toolbar 4 for Internet Explorer, but Google uses many other values: "C" for Google Chrome, "D" for Google Desktop, "B" for Google Toolbar for Firefox, "U" for Google Pack. "ADBR" is a code that identifies the distribution channel. "This correlates to how the user got the software (ie. they downloaded it by itself vs. it came pre-installed on their new computer vs. it came with a partner's software)." "c" is a value that tells Google if someone was already a Google user.
The library sends Google two other interesting values: install cohort (the country and week of the user's installation event) and first search cohort (the country and week of the user's first Google search). The week is measured as the number of weeks since Feb 3, 2003. For this example, "US239" informs Google that the user performed a search from the US in September 2007.
It's interesting to notice that Google measures the success of a campaign that promotes Google Toolbar, Google Chrome or other Google software by counting the number of Google searches.