When creating regular and smart tracking links you specify:
Target URLs and deep links support pattern processing. Patterns allow you to pass parameters from Click URLs to target URLs and deep links. This may be helpful when you need to supply additional information (user ID, campaign data, etc.) to a new installation or a deep link, respectively.
Patterns are somewhat similar to macros: a pattern {GET_VAR_NAME} in a target URL or deep link is replaced with a value of GET parameter named GET_VAR_NAME from a Сlick URL. In other words, if a Сlick URL looks like:
https://trk.mail.ru/c/123?screen=foo&username=bar
and in the target URL or deep link you wrote a pattern:
http://myproject.com/{screen}/?user={username}
then the user will be redirected to
http://myproject.com/foo/?user=bar
If the required parameter has not been supplied to the original URL, the pattern will be replaced with a blank line.
You need to create a pattern for target URL and deep link when you add a regular or smart tracking link