E-mails from readers have revealed how confused they are about the self plus one option that’s being added to the Federal Employees Health Benefits Program with the 2016 plan year. It’s no wonder; this is a big change. Since the FEHB program began in 1960, there have only been two enrollment options: self only and self and family. While the “self” part of the new option is clear, what’s not so clear is the “plus one.”

The short answer is that self plus one allows you to cover yourself and one eligible family member. The definition of eligible family members hasn’t changed. If you choose self plus one, the second person can either be a spouse or either a child up to age 26 (or a child age 26 or over who is incapable of self-support because of a mental or physical disability that existed before age 26). Note: Only legally married spouses, which includes same-sex spouses, are considered eligible family members under any FEHB enrollment. That excludes domestic partners.

Self plus one should be attractive to many people who have only one eligible family member—a married couple with no eligible children, most commonly—but who traditionally have had to pay for family coverage at the same premium rates as persons with multiple covered family members.

If you are an employee and choose a self plus one enrollment during the 2015 open season, your enrollment change will be effective with the first pay period of 2016. Open season enrollments for annuitants are always effective on the first day of the year following the end of the open season.

Moving from one enrollment category to another can also be done between open seasons if your situation is one of those included in OPM’s Table of Permissible Changes—for example, when a single person gets married. If you choose a self plus one enrollment outside of open season, your enrollment change will be effective on the first day of the first pay period following the one in which you make the change.

While it’s logical to assume that the premium rates for self plus one will be substantially more than for self only enrollees, it remains to be seen if they will be substantially less than those for self and family. The premium rates won’t be known until open season begins this fall.

Here’s a piece of advice from OPM: “You should determine your eligible family members and decide which enrollment type is best for you. A self plus one enrollment type will cover you and one eligible family member. During open season 2015, you will want to pay close attention to the benefits and rates in both the plan you currently have and other FEHB plans available to you. OPM will release materials in advance of open season to help you make this important decision.”