Insurance Plans - What's Covered
This seems like a simple question but there isn't an easy answer. Every plan is different, and the reasons might surprise you.
For individual plans, or plans that you buy yourself, the coverage is determined by the group or company that you buy it from. Most plans cover all of the basics with varying levels of deductibles and co pays, but there are some plans that cut corners to save money, and may leave you uncovered (for example, a plan that excluded pregnancy or cancer coverage). Be sure to check the coverage before you purchase an individual plan.
For group plans that you get at work, especially with larger companies, the coverage is probably determined by your employer rather than the insurance company. In fact, many large companies are Self Insured, and just contract with an insurance company to do the paperwork and negotiate fees.
So suppose that you go to the doctor to get a tattoo removed, and the insurance company from your job at MegaCompany Inc. tells you that it won't pay the bill because it's not covered. The underlying reason is probably because MegaCompany decided not to cover tattoo removal.
It's possible to buy insurance that covers everything, and the insurance companies would probably love to sell it, but the cost would be enormous.