[CQ-Contest] What is "one radio"?

[Sebastian Delmont]

Many contests include a "Classic" overlay that is defined as, among other
things, the requirement to "use only one radio".

How does this affect the use of radios with multiple reception units?

Does this rule prevent SO2R operation when you use a single radio, with a
single transmitter but able to listen to more than two signals at once?

Or was this rule meant to prevent this kind of operation, but got obsolete
and was never revisited?

73 de KI2D