I recently came across with the catalogue where the wire sizes and their specifications are listed. In that, there was a specification "current carrying capacity - 2 wires single phase". What does this specification mean? I normally know the meaning of current carrying capacity but this what does this "2 wires single phase" exactly mean. Whether it says that one wire is connected to line and another to neutral in single phase so that two wires provide that specified current rating or it says two wires in line and two wires in neutral are to be connected to achieve that provided current rating?
“Two cables single phase” means they’re giving the capacity for two wires pulled together through an enclosure and carrying the same current. That’s usually hot and neutral, but it could be two hots or two neutrals (though there are often other code issues there).
Why is that needed? The ampacity limit comes from the maximum acceptable temperature rise under specific conditions. These wires are used to carry current in a conduit or raceway. So that’s how they’re rated: how much current can we carry in a specific reasonable situation before the temperature rises too much. (Note that the cable tray case cools better, so can often handle more current)
The various electrical codes specify how to get from these kind of ratings to ones with more wires, mixed sizes, etc.
The limiting factor in conduit isn't the surface area of the wires (which goes up as the square root of cross section) but rather, the surface area of the conduit. So more wires in conduit reduces the ampacity of all of them.
This is reflected in NEC 310.15B3a.