The primary user is the person responsible for a device's security and maintenance. In many cases this is unambiguously the person who is the sole user of a device, but sometimes it will be a system administrator, or one user in a collection of occasional users.
It is necessary to specify a primary user's SUNet ID during device enrollment in order for a device to be associated with that user in MyDevices. A Mac OS or Windows device can have any number of secondary users, whose enrollment answers are saved, but ignored for compliance purposes.