Environment variables to sent to Testlab as test case parameters.
Test cases in Testlab can include parameters in the content as ${PARAM1} formatted tags. Normally, these tags are set and replaced with values set during the test case execution or execution planning phase. By listing variables to this setting makes it possible to send values of environmental variables from Jenkins as test case parameters to Testlab.
For example, test case(s) in your Testlab project might have a parameter titled ${BROWSER}. When running automated UI tests in your Jenkins, enter a value "BROWSER" to this setting and ensure that your job sets an environmental variable BROWSER to some sensible value. This way running the job sends and sets the ${BROWSER} test case parameter value to all run tests to a value matching to the environmental variable.
Probably the easiest way to set environment variables is to use a fitting plugin.