Since the end of 2015, the MCS-150 form is used to update a company's registration with the FMCSA. Prior to this date, it was also used to obtain a new USDOT number. This form's designation itself doesn't mean much. The "MCS" stands for "motor carrier services" and the 150 is simply a designation.
The USDOT is used as a unique identifier to collect census data regarding the operation and to monitor the company's safety information. All violations found at roadside inspections are recorded to the operating carrier's USDOT number.
All interstate carriers, interstate intermodal equipment providers, and any interstate or intrastate carrier requiring a hazardous material safety permit is required to submit an updated registration to the FMCSA at least every other year. The update is required in a specific month based on the last two digits of the company's USDOT number. The last digit determining the month to update and the second to last determining whether to update in even or odd years. Not providing this update can result in the deactivation of the USDOT number and the operation is then prohibited from operating commerical motor vehicles.
It may be wise to provide updates in addition to the mandatory every other year update. When a company is growing, an interim update will allow a better comparison between operations for the Crash and Unsafe Driving CSA BASICs. These two BASICs take into consideration the miles traveled and the number of power units operated provided on the MCS-150. The Unified Registration system also uses the vehicle count as a starting point to determine fees. A carrier that is reducing in size may benefit from an interim update to ensure the correct UCR bracket is used.