Most commercial commodity specifications for hydrocarbons such as gasoline, kerosene, Diesel fuel, jet fuel, naptha, mineral spirits, etc are fairly broad. RP-1 is kerosene that meets some particular specifications that are important for use as a rocket fuel, but not so important for burning it in gas turbines or diesel engines. The specifications for RP-1 are outlined in MIL-R-25576.
Part of the reason for hydrocarbon rockets using a variant of kerosene is a result of the way that rocket development in the 1940s and 1950s had a substantial military focus on missiles. Much of this is focused on in Clark's Ignition, and Clarke was a Navy rocket scientist. It was desired to use jet fuel in missiles and aircraft rockets because this would simplify logistics for the military. However, jet engines are not very picky (related: Why the M1 Abrahms tank engine can run on such a wide variety of fuel), and so the specifications for jet fuels are quite broad and preoccupied with availability, safety, temperature stability/freeze resistance, and practicality at high altitudes (which requires a vapor pressure that is not too high). Eventually it was decided to create a spec for hydrocarbon rocket fuel that would be usable as jet fuel as well, even if most jet fuel wasn't usable for rockets.
The RP-1 spec includes some of the following requirements:
- Very low sulfur (Damages hardware at high temperatures, makes pollution, catalyzes polymerization
- Very low olefins and aromatics (these tend to polymerize in regenerative cooling channels, and also gum up hardware in missiles that are stored for long periods, though that isn't a problem for civil space launchers
- More branched-chain alkanes than normal (improves thermal stability when regenerative cooling)
- Very low impurites, mostly to avoid destroying turbopumps
- A pretty narrow range of molecular weights, to keep the lubricating properties consistent and to keep the fuel from changing due to separation or differential evaporation
Many of these goals can also be achieved by using a pure substance, or a mixture of pure substances, for a rocket -- propane, toluene, diethyl cyclohexane, 1-methyl-1,2-dicyclopropyl cyclopropane, for example, have all been studied. However, most pure substance hydrocarbons (other than methane, ethane, and similar light gases) are much more expensive and less available (especially under military logistics) than a petroleum mixture.