Explanation
A random split activity is an automation studio activity that randomly splits a data extension into multiple groups based on percentage or number of records. It can be used to create test groups for different messaging strategies or to segment a large audience into smaller batches for sending. It can handle up to 12 groups and up to 50 million records per group.
References:https://help.salesforce.com/articleView?id=sf.mc_as_random_split_activity.htm&type=5