That is a device, not an extension. You need to ask the teacher to clarify what they mean by an extension. Extensions, in Asterisk terminology are not the entities that can place calls.
In normal configurations,where you are using real phones, with their own IP addresses type=peer is the only setting of type you should be using. The distinction is as to whether only the IP address is used tot associate calls with the device or whether the user part of the from address is used, and preferred.
The ability to place calls is determined much more by the dialplan, e.g. think about how external callers are normally allowed to place calls to locally terminated extensions, but not to other external parties.