Device/user mode question

i’m new to freepbx (first post)which I’m using with asterisknow. i have configured 3 accounts in device mode and registered without any problems on softphones and ip phones.

I’m trying to develop an understanding user and device mode.
when enabling device&user Mode, making an account in user mode and linking it to a device, I can’t get it to register or work out how to use on other or different devices. can someone explain how to register a softphone or ip phone so a user can log on and off it?

thanks in advance.