From LimeSurvey Manual
Randomization group name (random_group)
It places the questions into a specified randomization group, all questions included in the specified group being displayed in a random order to the survey respondents.
You can find a sample survey using randomization group name in Expression Manager sample survey.
Just enter any string you like (for example: 'group1'). All question which have set the same string within the randomization group name box will have their place in the survey randomized (=randomly exchanged among each other).