Intermediate metal conduit, or IMC, is a thinner, lighter-weight version of rigid metal conduit and is approved for use in all of the same applications as RMC. Because IMC is lighter and easier to work with than RMC, it is more common in new construction. One of the biggest benefits of IMC is that it is permitted to be installed in concrete, in contact with the earth, and in areas that are subject to severely corrosive environments.
EMT cannot be buried in the ground because the ground would start to degrade the thin walls of the conduit much quicker than IMC or RMC. Therefore many direct-burial applications utilize IMC for longevity, as well as the shallow burial depth requirements in the National Electrical Code. (NEC 2017 Table 300.5)
As with EMT, there are different types of IMC that can be used depending on the installation environment. They typical IMC you’ll run across will be galvanized steel IMC which is used in less corrosive environments, however there is a stainless steel version that must have matching stainless fittings and accessories – and when installed correctly can last much longer than galvanized. (Read 342.10 of the 2017 NEC for more on permitted uses of IMC)