About Device & User Mode
By default, FreePBX operates in "Extension Mode." In Extension Mode, devices (such as phones) and users (people) both use the same identification number, i.e. their extension number. Each Device (Phone) is assigned one User (Extension) and they cannot be changed without accessing the FreePBX User Interface. A single module, entitled "Extensions" will manage the device settings and the user settings.
FreePBX can be switched to operate in "Device & User Mode" by making changing the "User & Devices Mode" option in the Advanced Settings Module.
Device & User Mode will cause the Extensions Module to disappear and to be replaced with two separate modules, one entitled "Devices" (i.e., phones) and the other entitled "Users" (i.e., extensions).
The Devices module sets up device related settings, i.e. the Device Number (which should be a number that is different from any extension number), the Device's Name (usually the location of the phone), the password the Device uses to register with the PBX, etc. The Users module sets up the User related settings, such as the extension number, the person's name, whether that person has voicemail, the voicemail password, etc.
There are two main advantages to Device & User Mode.
First, two devices (i.e., phones) can share the same extension number. For example, if a manager has an office on two different floors, he might want to have the phones in both offices have the same extension number, so that regardless of which office he's in, a call to the same extension number will ring both phones. Both phones can also receive intercom calls to the same extension number, and both phones will show when he has a voicemail. This can be accomplished by setting up each of his phones as a Device, setting up one User (i.e. the manager), and then assigning the User to both devices.
Second, extensions can roam from one device to another. For example, suppose that John and Jane both work in the same office. John works Monday, Wedneday, and Friday. Jane works on Tuesday and Thursday. When John and Jane come to work each day, they could pick up any phone that's not in use and "login" using a feature code, thereby causing their extension # to be registered to the phone that they are in front of, so that they can start receiving and making calls. Before they leave, they can log-out using another feature code. If they both happen to come in on the same day for some reason, either can use any other unassigned phone.
Comments about User and Device mode should be taken to the forums for community discussion.