The Pentagon has issued a comprehensive manual detailing protocol for service members undergoing a sex-change transition that includes extended time off, getting a commander’s approval for medical treatments and a prohibition on living a “preferred gender” lifestyle on base.

In the step-by-step process laid out by the Department of Defense, a service member must first secure a medical diagnosis from a military medical provider. Next, the service member must notify his or her commander of a diagnosis "indicating that gender transition is medically necessary."

The service member’s commanding officer plays a critical role throughout the process -- first having to approve government-funded medical treatment for the sex change and later overseeing when the individual is able to return to service.

There are currently between 1,300 and 1,600 transgender service members in the active force, according to a study by Rand Corp.

The directive -- which seeks to address the "unique challenges" of sex changes in the military -- prohibits personnel from living their “preferred gender” lifestyle, or “Real Life Experience,” on a military base with other service members until the transition is complete.

Real Life Experience is defined as "The phase in the gender transition process during which the individual commences living socially in the gender role consistent with their preferred gender."

The 18-page manual takes effect on Oct. 1. The directive makes clear that "Effective immediately, no otherwise qualified service member may be involuntarily separated, discharged or denied re-enlistment or continuation of service, solely on the basis of their gender identity."

Defense Secretary Ash Carter announced last month that transgender individuals will now be able to openly serve in the U.S. armed forces.

“This is the right thing to do for our people and for the force,” Carter said in June. “We’re talking about talented Americans who are serving with distinction or who want the opportunity to serve.

"We can’t allow barriers unrelated to a person’s qualifications to prevent us from recruiting and retaining those who can best accomplish the mission," he said.

Last summer, Carter called the Pentagon's current regulations regarding transgender service members "outdated" and said they were "causing uncertainty that distracts commanders from our core missions."

"At a time when our troops have learned from experience that the most important qualification for service members should be whether they're able and willing to do their job, our officers and enlisted personnel are faced with certain rules that tell them the opposite," Carter said in 2015.