The territorial designation is usually a description of the place with which the peer or peeress has a connection or where he or she has a residence. All peers below the rank of earl must be described in his or her letter patent as ‘of ———‘ (somewhere).
Some peers are created with a shortened form of their territorial designation as an integral part of their title, such as Lord Archer of Weston-super-Mare. It should be noted that his full title, including his territorial designation, is Lord Archer of Weston-super-Mare, of Mark, in the County of Somerset. This territorial designation helps to avoid confusion with another life peer with the same surname, Lord Archer of Sandwell (Lord Archer of Sandwell, of Sandwell, in the County of West Midlands).
Even when there is no cause for confusion, a peer may decide to incorporate a territorial designation into his title, such as Lord MacLaurin of Knebworth.
Other peers do not use their territorial designations as a part of their title, for example Lord Attenborough is simply Lord Attenborough, not Lord Attenborough of Richmond-upon-Thames, although he is often described as such in the Press.
All works of reference make it quite clear whether the territorial designation should or should not be used for any particular title.