The Circuit describes the connection of the corresponding Phone Number Range(s) to the PSTN. The Circuit entry contains the attributes for the circuit through which associated Number Ranges run. The Circuit information is typically gathered during the preparation phase of a migration to Lync or Skype for Business project. Circuits may be entered into Provisioning Solutions for informational purposes, however, they are not utilized by Provisioning Solutions. Circuits are identified by their Circuit Identifiers, which are typically provided by the carrier. The Site refers to the physical location of the Circuit and not necessarily the location(s).
Example:
An office in London Westminster uses Site Code of LDN_WM, and employees there are tagged with that specific code in Active Directory, to identify where they are located. A circuit terminating in that office would then use LDN_WM as the Site Identifier.
If instead, the Westminster office uses a circuit from the London central office which uses Site Code LDN_C, the circuit's Site Identifier would be LDN_C.