It is easy to confuse these 2 terms on SIMSafe as they may sound like the same thing, but there is a big difference. A username is simply the name you associate with a device. It can be whatever you choose.
“Devices” sit at the next level down. The Device identifier is always the calling number of the SIM (MSISDN).
An example of this relationship is as follows:
SIM test01 +46(0)719000xxxxxx
In the above example, SIM +46(0)719000xxxxxx is locked to username SIM test01. The username can be anything you please, but the device will never change.