Yes, it's only relevant in the US, although in the EU the equivalent (more or less) rule is EU Regulation 261/2004. Technically it no longer exists.
From the FAA's FAQ:
The term "Rule 240" refers to a rule that existed before airline deregulation. There is no longer an actual Rule 240. The term, as it is now used, refers to each airline´s "conditions of carriage" policy. You would need to contact the airlines to obtain this.
And then there's also a relevant Wikipedia page:
Federal Aviation Administration Rule 240 mandated that an airline with
a delayed or canceled flight had to transfer passengers to another
carrier if the second carrier could get passengers to the destination
more quickly than the original airline.
The original rule, referring to a federal requirement before airline
deregulation in 1978, is long-obsolete; however, the major US
airlines have filed "conditions of carriage" with the U.S. Department
of Transportation guaranteeing their similar provisions. These
provisions vary from airline to airline, and generally apply only to
delays that are absolutely the airline's fault, such as mechanical
delays, and not to "force majeure" events such as weather, strikes, or
"acts of God".