From LimeSurvey Manual
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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 ExpressionScript 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).
Preview To preview the questions use the preview survey instead of the preview question group function, as the second has been reported to not show the questions in a randomized order.