Buried in an earlier posting I talk about medigap vs. IBM medical offerings but it is worth repeating because it is very confusing. If you are using original medicare there is government defined secondary insurance available for purchased from private insurers typically referred to as medigap insurance to cover your deductibles and the copays. There are 12 different types of plans defined by the government and each is designated by a letter A,B,C, M, N and so on. A private insurer offering plan "M" must structure it according to "M" components. Sometimes the private plans are called supplemental insurance. To find out what companies offer medigap plans in your zip code go to medicare.gov. Incidently, it makes no sense to pay more for a given plan - pick the cheapest private insurer for the plan you want. The plans offered through AARP (which are usually United Healthcare plans) are often priced well but not always so do comparison shopping!
NONE of IBM medical insurance options are medigap plans. That is why IBM medical plans are often referred to as secondary insurance. By law, you are only allowed to buy one medigap plan. Since IBM medical insurance is NOT a medigap plan you can buy IBM's medical/prescription drug coverage and also buy a medigap plan. Do this if you want IBM's drug plan but want better medical coverage than IBM's medical plan. It is unlikely that the IBM medical component of the coverage will be used but it can happen. There are services like acupuncture that are covered by IBM but not covered by a medigap plan.
Do not believe the IBM service center if they tell you that you cannot buy a medigap plan if you buy an IBM medical plan. It is not true.