What the regulation says
Under 49 CFR 387.311, a Form BMC-91X on file represents either the full federal security limit for the carrier or the specific limit shown on the face of the form. So a BMC-91X can certify full coverage or any primary or excess layer of it. When more than one insurer stacks coverage to reach the required limit, each insurer files its own BMC-91X for its layer.
If a BMC-91X reflects aggregation, the certificate must show whether that insurer's coverage is primary or, if it is excess, the amount of underlying coverage and the maximum limit. A BMC-91, by contrast, always represents the full minimum limit by itself.
Why it matters to a carrier
For a carrier the practical goal is the same with either form: a valid liability filing has to be on record before authority goes active, and it has to stay on record without a gap. Many insurers file the BMC-91X even when one policy provides the full limit, because the form covers both situations. Your agent and insurer choose the form; you do not need to request one or the other.
Where coverage is layered, every layer has to remain in force. If the primary insurer cancels and nothing replaces it, the excess filing alone may not satisfy the requirement.
Checking your record
You can look up your carrier's insurance filings on FMCSA's public licensing and insurance system. Check it after binding, and again if you change insurers, to confirm the new certificate was accepted and there is no gap between the old filing and the new one.
Frequently asked questions
Is a BMC-91X better than a BMC-91?
Neither is better. Both prove liability coverage to FMCSA. The BMC-91X can also show a single layer of coverage, which the BMC-91 cannot.
Does a layered program need more than one filing?
Yes. Each insurer in a layered program files its own BMC-91X, and under 49 CFR 387.311 each certificate shows whether that layer is primary or excess.