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2.1. ezma/use allows to use general marketing automation features
2.2. ezma/user allows currently logged in user to create a new user accounts in the Net-Results system
3. DO NOT DELETE USERS, DISABLE THEM
if there are objects (Campaigns, Segments, etc.) that are assigned to a specific user's Role, that user being deleted would make those objects unavailable for anyone else to see. It would be better to disable the user (there is a checkbox on the User Administration screen for each user which designates them as enabled/disabled) rather than delete them, but unless a customer is a heavy user of Roles and object ownership it usually doesn't make a whole lot of difference.