The industrial scale was a system used to rate the extent of industrial development on different planets.
The Federation utilized an industrial scale system during the 23rd century that was scaled by either letters or numbers.
During the late 2260s, the planet Sigma Draconis III rated letter "B", and the planet Sigma Draconis IV rated letter "G". (TOS: "Spock's Brain") Scalos was rated number "7". (TOS: "Wink of an Eye")