Nomenclature
R2 "signalling" is consistently "signalling" (with a double ell) in standardization documents, which are consistently written using British spelling, as the standard was not adopted for wide use in the United States.
R2 is mnemonic for Region Two signalling to differentiate it from R1 signalling, the North American MF signalling. Here region number two was envisioned in the 1960s to be Europe and region number one was envisioned in the 1960s to be the Bell System in North America. In practice the term R1 is rarely used for North American MF signalling, except to contrast with its European contemporary, R2. Later in the 20th century, use of R2 signalling spread beyond Europe to all regions of the globe, including Mexico on the North American continent. This spread is due largely to European telecommunications manufacturers selling their older equipment designs at a discount to developing countries while selling their higher-speed, higher-density post-R2 SS7 equipment at higher prices in more-industrialized countries. There is even sparse use of R2 in Canada, which is largely under the influence of Bell System standardization in North America.
See also compelled signalling.
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