Although these terms can be used interchangeably, the "pocket" is the space dissected and where the breast implant is placed. The "capsule", on the other hand, is the scar tissue that subsequently forms around the breast implant. Hope this helps.
The pocket is the space dissected for the implant. The capsule forms on the inside of the pocket (like a lining) as you heal. If this capsule shrinks, you develop a contracture and distortion and hardness of your breasts. So the pocket is not the same thing as the capsule.