Please note: There is not a system of "preference" between users who belong to a Department as opposed to a Group (or perhaps belong to both).
Departments do not override Groups, and Groups do not override Departments. The permissions that you grant to a user (or collection of users) within a Department or within a Group are simply "on" or "off". One permission does not cancel out another.
Tips / FAQ
Basic human rights should only have permissions that it's OK for them to have over managers and CEOs.
If one wants a department or group to be able to barge each other, create a separate department or group for that. They can barge...each other. Do not include managers in this dept/group.
Managers are in a separate group, and only this group has the barge permission over current groups: basic human rights.
A common setup is for ACD Managers to have the barge permission over *only* the current group: ACD Agents, and simply remove the CEO from the ACD Agents group.
Basic rule: don't try to prevent people from doing things, or block things. Rather, look at it like granting people only the basic permissions they need.
Query: What if a user is a member of 2 different groups that have conflicting permissions (1 group allows transfer from and to while the 2nd group does not have the permission)?
Answer: Permissions are cumulative. As long as you have a permission in at least *one* group, you have that permission. Period.
User Permissions vs. Group Permissions: Transferring
User permissions - Transferring:
Tends to affect calls that do *not* involve another user.
Transfer calls from my extension: Transfer calls from yourself to park/hold, for example.
Transfer calls to my extension: Retrieve from park/hold back to your phone.
Group permissions - Transferring:
Takes effect over the "current groups" below it. E.g. I can transfer to coworkers in Current Group: Support, but I cannot transfer to people in Available Group: Sales.
Transfer calls from others extensions: Take away (steal) a call from a coworker - either to oneself, or to someone else (assuming you *can* transfer to others extensions)
Transfer calls to others extensions: Transfer a call to a coworker. Either from oneself, or from someone else (assuming you have *permission* to transfer a call from someone else).
Reference, further information
Please also see:
Permissions Definitions. What different permissions do.
Departments. Useful for visually sorting in extensions in the HUD contact list.