The MCS-90 is not insurance in the way most people think. It's a federal endorsement bolted onto your liability policy that promises to pay the public if you cause bodily injury or property damage while hauling regulated cargo and can't otherwise pay.
The FMCSA requires it for interstate for-hire motor carriers of non-exempt commodities. Minimum limits are $750,000 for general freight and higher for hazmat.
Where it stops: the MCS-90 responds to third-party public claims. It does not cover your truck, your trailer, your cargo, or your driver. Those need their own coverages — physical damage, cargo, occupational accident or workers' comp.
The MCS-90 is also a 'suretyship' — if the insurer pays a claim under it that would normally be excluded, they can pursue you for reimbursement. Do not think of it as a safety net. Think of it as the FMCSA's guarantee to the public, funded by your policy.
If you're setting up new authority, ask your agent to confirm the MCS-90 is on the policy and that the BMC-91 (or 91X) filing is submitted to the FMCSA. Without that filing, your operating authority won't activate.