Authentication server for new Thread devices and the authorizer for providing the network credentials they require to join the network. A device capable of being elected as a Commissioner is called a Commissioner Candidate. Devices without Thread interfaces may perform this role, but those that have them may combine this role with all other roles except the Joiner. This device may be, for example, a cell phone or a server in the cloud, and typically provides the interface by which a human administrator manages joining a new device to the Thread Network.
A device that is capable of becoming the Commissioner, and either intends or is currently petitioning the Leader to become the Commissioner, but has not yet been formally assigned the role of Commissioner.
A human-scaled passphrase for use in authenticating that a device may petition to become the commissioner of the network. This credential can be thought of as the network administrator password for a Thread Network. The first device in a network, typically the initial Leader, MUST be out-of-band commissioned to inject the correct user generated Commissioning Credential into the Thread Network, or provide a known default Commissioning Credential to be changed later.
A device to be added by a human administrator to a commissioned Thread Network. This role requires a Thread interface to perform and cannot be combined with another role in one device. The Joiner does not have network credentials.
An existing Thread router or REED (Router-Eligible End Device) on the secure Thread Network that is one radio hop away from the Joiner. The Joiner Router requires a Thread interface to operate, and may be combined in any device with other roles except the Joiner role.
Information extracted from the Thread Whitepapers available at threadgroup.org