What is the difference between dual plane and under muscle placement of breast implants?

In essence, all subpectoral breast augmentations are dual plane, in that the implant sits both behind the pectoralis major muscle in the upper inner part of the breast, and behind the breast itself in the lower outer part of the breast. There is no pectoralis major muscle coverage in the lower outer part of the breast so all subpectoral augmentations are in essence dual plane. We use the terms dual plane one, two, and three to refer to the level at which the muscle is separated from the breast. All subpectoral augmentations are at least a dual plane one, meaning that the pectoralis major muscle has been released across the entire length of the inframammary fold. We use dual plane two to refer to a situation where the muscle is released up to about the lower border of the areola. Dual plane three is a release to just above the top of the areola. Which level of release you choose depends on: 1) what the patient is starting with, 2) the implant you are using, and 3) the outcome you are trying to achieve. 

These are surgical terms related to various classifications. It is uncommon in aesthetic practice for an implant to be completely covered by muscle. In fact in the lower portion of the chest wall the muscles are virtually non existent and the actual cover in this region is a very thin fascia and so in my opinion complete muscle cover is impossible. In Dual plane the implants are placed under the muscle in the upper portion where the muscle exists and above the chest wall and under the breast (sub glandular) in the lower portion.