An MC (Motor Carrier) Number is the specific identifier FMCSA assigns when it grants a company for-hire operating authority — it's what most people mean when they informally say "docket number," since MC is the most common of the three possible prefixes (the others being FF for freight forwarders and MX for Mexico-domiciled carriers). An MC number is completely separate from a USDOT number: the USDOT number is issued for basic safety registration and applies to virtually every commercial motor carrier, while an MC number specifically reflects that the company has been granted authority to haul for compensation — a carrier can hold an active USDOT number without ever having an MC number if it's never applied for for-hire authority, as with a private carrier moving only its own goods. A single company can accumulate multiple MC numbers over its history, and it's the associated authority status — not just the existence of the number — that determines whether the company can legally operate as a for-hire carrier or broker today. Checking that an MC number's authority is Active, not Revoked or Inactive, is one of the most basic due-diligence steps before doing business with a carrier or broker.
A company can have USDOT number 3136906 and MC number 1042318 — two entirely different identifiers serving two different regulatory purposes.
Shown as the 'MC' ID chip at the top of carrier profiles, and as the primary identifier (plate graphic and ID chips) on every broker profile.