In general...

Companies that operate as “for hire” carriers (for a fee or other compensation) that transport passengers or federally regulated commodities, or arrange for their transport, in interstate commerce are also required to have interstate operating authority.

The Reach of Operating Authority
FMCSA operating authority is also referred to as an "MC," "FF," or "MX" number, depending on the type of authority that is granted. Unlike the USDOT Number application process, a company may need to obtain multiple operating authorities to support its planned business operations. Operating Authority dictates the type of operation a company may run, the cargo it may carry, and the geographical area in which it may legally operate.

Financial Responsibility and Exemptions
All of this also dictates the level of insurance/financial responsibilities a company must maintain. Carriers not required to have operating authority include Private carriers, "for-hire" carriers that exclusively haul exempt commodities (cargo that is not federally regulated), or carriers that operate exclusively within a federally designated "commercial zone" that is exempt from interstate authority rules. A commercial zone is, for example, a geographic territory that includes multiple states bordering on a major metropolitan city, such as Virginia/Maryland/Washington, DC.