Millions of people age 65 and older receive healthcare coverage through Medicare, the government-run health insurance program for seniors. However, Medicare doesn't cover everything, and accordingly, nine out of 10 seniors on Medicare have some form of supplemental coverage, such as a Medigap plan.

There are currently 10 different Medigap plan types — plans A, B, C, D, F, G, K, L, M and N — each of which provides different benefits. If you will soon be eligible for Medicare and are considering supplemental coverage, Medigap Plan B is one of the plans you might want to explore. Here's what you need to know.

What Is Medigap Plan B?

First, it's important to make a distinction between Medicare Part B and Medigap Plan B, because they are two entirely separate forms of coverage.

Original Medicare is split into two parts: Part A, which provides hospital insurance, and Part B, which provides medical insurance for medically necessary and preventive services. However, Medigap Plan B, as the name suggests, is designed to fill coverage gaps left by Original Medicare. To get a Medigap plan, you must have Medicare Parts A and B. Medigap plans are offered by private health insurers and can offset out-of-pocket healthcare costs such as copays, coinsurance and deductibles. As with Medicare Part B, you'll have to pay a monthly premium for any Medigap policy you choose.

Both policies will pay a designated amount for covered healthcare services. Typically, when you need to receive treatment or visit a doctor, Medicare will pay first for covered services, followed by your Medigap plan.

All Medigap supplement plans, from Plan A to Plan N, cover Part A coinsurance and hospital costs up to an additional 365 days after Medicare benefits are completely used. However, Plan B has several benefits that vary from the nine other Medigap plans.

Medigap Plan B offers several benefits, including covering the costs of your:

• Part B coinsurance or copayment.

• Part A hospice care coinsurance or copayment.

• Part A deductible.

• First three pints of blood if you need a transfusion.

Medigap Plan B doesn't cover:

• Coinsurance for skilled nursing facility care, which is the share you must pay for covered services. Typically, you're responsible for 20% of the costs of covered healthcare services, and your plan will pay the remaining 80%.

• Part B deductible, which is the amount you pay out of pocket for covered healthcare services before your insurance kicks in and starts to pay.

• Part B excess charge. This is the difference between the legally-approved amount a doctor is allowed to charge over the Medicare-approved amount for covered healthcare services.

• Foreign travel exchange. This is emergency coverage if you get sick while you're traveling abroad. Typically, Medigap plans come with a lifetime coverage limit of $50,000 for foreign travel emergency care.

Additionally, there's no out-of-pocket limit for Medigap Plan B. In comparison, Plans L and K have out-of-pocket limits that total $3,110 and $6,220, respectively. With no out-of-pocket limit for Plan B coverage, you could face higher out-of-pocket costs because there's no point at which the Medigap plan will pay for 100% of covered services. However, since Medigap is supplemental coverage and is only used to fill Medicare coverage gaps, it's possible that between both plans you could have very few out-of-pocket expenses, especially if you're healthy and don't visit a doctor that often.

How to Get Medigap Plan B

You qualify for Plan B coverage if you're age 65 or older and are eligible for Medicare.

You can get a Medigap plan during the six-month open enrollment period, which typically begins the first month you have Medicare Part B coverage. It's best to get a Medigap policy during open enrollment because you'll often have more plan choices and may be able to find more plans that fit your budget.

If you don't choose a Medigap plan during this time, you may not be able to purchase this coverage later, or it may be too expensive, since your health history will be more of a factor in the underwriting process. However, some states offer what's known as Medicare Select, another type of Medigap policy that's available outside of the traditional open enrollment period. If you choose to purchase a Medicare Select policy, within one year you'll be allowed to switch to another Medigap plan if you aren't satisfied with this coverage. A Medicare Advantage plan, which private insurers also offer, is another alternative. These plans, also known as Medicare Part C, replace Original Medicare and sometimes offer more comprehensive coverage beyond what traditional Medicare provides, such as dental and vision coverage.

Is Medigap Plan B Coverage Right for You?

So how do you know if Plan B coverage is right for you?

First, look at all of the benefits across the 10 Medigap plans and compare them. If you need help with paying your Medicare Part B deductible (for medical insurance coverage) or will be living abroad most of the time, then Medigap Plan B won't be the best fit for your coverage needs because it doesn't cover the cost of this deductible and doesn't include foreign travel coverage.

Next, you'll also have to do some basic math to figure out if you can afford both the monthly Medicare Part B premium and the Medigap Plan B monthly premium. It's important to mention that if you have Social Security, your Medicare Part B monthly premiums are usually deducted from your monthly Social Security check. However, if you live on a fixed income and mostly rely on Social Security to cover expenses, then Plan B Medigap coverage may be too cost-prohibitive.

You'll also need to compare potential out-of-pocket costs for Part A and B coinsurance, copayments and deductibles to your potential Plan B monthly premiums. If the monthly premium for Plan B coverage is considerably less than the potential out-of-pocket costs you'll incur for Part A and B coinsurance, copayments and deductibles, then it likely makes more financial sense to purchase this Medigap plan.

Not every insurance company offers Medigap Plan B, and in some cases, it may be too expensive to get this coverage or more affordable to go with another Medigap plan. As you assess your healthcare coverage options in retirement, carefully consider plan benefits and monthly costs to decide whether a particular plan aligns with your healthcare needs and budget. While Medicare includes both hospital and medical insurance, not every service is covered, and you still may have out-of-pocket costs for certain treatments or services. This is where Medigap coverage can be so valuable. Either way, take the time to do your research and carefully weigh all of your options as you decide whether Medigap Plan B or another supplemental plan is right for you.