You are comparing apples to oranges with a CSL policy and an Umbrella policy since CSL is in regards to car insurance Liability limits while an Umbrella policy is an excess Liability policy that is used if your car or homeowner's policy limits are used up due to claims or lawsuits.
Combined Single Limit (CSL) coverage for 1 million dollars would strictly be under your car insurance policy and pays out only for covered losses under that policy. An Umbrella policy for 1 million dollars will cover you if you have claims against you or are sued and have to pay more than the coverage limits under your auto and/or homeowner's policies.
When you have a combined single limit it means your car insurance company combines both Bodily Injury Liability and Property Damage Liability insurance under a single limit. The insurance company would pay up to the stated limit on a third party claim, regardless of whether the claim was for bodily injury or property damage or for both.
Typically with a car insurance policy most people have split limits which is called so because the Liability insurance is split into 3 sections - Bodily Injury per Person, Bodily Injury per Accident and Property Damage. So a split limit could be 25/50/25 and your car insurance coverages would only be used up to those limits for bodily injury (per person and per accident) and property damage while if you have a CSL of $100,000 it could be divided up however was needed for the bodily injury and property damage claims that resulted from you being at-fault in a car accident.
An Umbrella policy, also called an Excess Liability policy, is designed to protect you if you are held personally responsible to another person for injury or property damage and your auto insurance and homeowners or renter's insurance (whichever policy the original claims have gone through) limits have been exceeded. Personal Umbrella insurance protects your home, property and assets by providing greater coverage than your auto insurance or homeowner's can provide.
An umbrella policy is supplemental coverage to be added onto the auto and home insurance that you already carry. Your auto insurance and homeowners policies are your first line of insurance coverage; if their limits are exceeded your umbrella policy would kick in. Which policy is used as primary depends upon how and where the damages where caused that you are liable for.
The main difference between CSL and Umbrella is that combined single limit is part of your auto insurance and is primary coverage if you are at-fault in an auto accident while your Umbrella policy is secondary and available to pay out only if your CSL Liability policy was exceeded.
Your umbrella policy would start where your other Liability insurance (such as CSL) ends. So if an action or judgment against you was for $800,000 and your car insurance Liability limits only covered you up to $400,000, the Umbrella policy would pay the claim above the $400k up to the limit of your Umbrella policy. If you had a CSL of 1 million dollars it may be hard to exceed, but normally auto insurance providers do not offer this high of Liability coverage.
Most car insurance companies require that you obtain both your auto and home insurance with them in order to purchase an Umbrella policy, and that is what our carriers at CarInsurance.com require. In addition, the insurance company may stipulate that your Auto or Homeowners Liability limits be at least a certain amount, such as $200,000 to $300,000. Umbrella policies are usually sold with a deductible that runs from $250 to $1,000.
If you would like this coverage through CarInsurance.com, purchase your auto policy and then we can add the coverage. Both Safeco Insurance and The Hartford offer Umbrella policies. We do not have a partner or affiliate that offers a stand alone Umbrella policy.