There's been several threads throughout the years that had talked about this, especially the GRS & Insurance thread. In a Nutshell, as GRS becomes included on more corporations insurance options, some companies are thinking forward and including top surgeries with bottom surgeries. BUT even though the big insurance companies (Cigna, Aetna, United, Blue Cross, etc...) have made panels and determined a structure on how to proceed for trans clients, the employers buying the plans to offer to you must opt in to such services first.
With that said, When an employer opts in to gender corrective services, you have to first select a plan that covers it, then go through a pre authorization process with a case manager to get things scheduled. If the plan covers breast augmentation for the transgender client, it will be filed under the same coding as if you had breast cancer and deemed medically necessary, instead of as an elective surgery. Elective means going to any cosmetic surgery center and going "oh my boobs aren't getting enough attention- can you make them bigger?"
So, what it means is that anyone who is applying for breast augmentation because of removing cancerous material, maiming/disfigurement, or (now) are trans and HRT did not provide adequate growth will be covered by insurance.