Remember that a tracking pixel is nothing but a little piece of html-code containing a transparent image (1 x 1 pixel dimension) that can be inserted in any banner, blog, e-mail or webpage that supports html.
Whenever the website, blog, e-mail or banner containing the tracking pixel is loaded in a web-browser, the tracking pixel will be requested from the server where it is hosted and each request written into a log-file.
By analysing the log file, we can count how many times the tracking pixel has been requested.
However, if both website A and B are using the same tracking pixel like this: visitanalytics.userreport.com/hit.gif ...
...we can't really tell if the request originates from website A or website B.
By adding a t= parameter like this:
...we can see if the impression should be counted towards website A or website B when counting the number of requests in the log-files.
In other words; the t-codes are responsible for allocating the right impressions to the right campaign / client.