In a breast augmentation or lifting breast ptosis by filling procedure the plastic surgeon creates a pocket where the implant will be placed. There are three placements for this pocket: under the mammary gland (sub-glandular), partially under the muscle (partial unders) or under the muscle (sub-muscular).    
Sub-glandular : The implant is placed over the pectoral muscles under the glands and tissue of the breast.  With this placement there is an increased chance of palpability (detection by feel or sight) of the implant, especially if you have little or no natural breast tissue present.  
This choice would benefit somebody who needs a quick fix of what is called a “mild ptosis” in other words, somebody who has mild sagging and wants to look fuller again, especially those women who nursed, or those women who are reaching their middle age and just need a quick lift.  If your Plastic Surgeon suggests you need a Breast Lift, you should listen, a sub-glandular breast augmentation might not be the right choice for you. 
For some patients, a sub-glandular procedure will give them a more natural look, since the implant will not be squeezed by the pectoral muscle. But there are patients that complain of feeling the implant, especially on or around the time of the menstrual cycle.  This is caused by the swallowing of the women’s glandular tissue, creating tenderness and increased sensibility of the breast tissue.

Sub-Pectoral only (Partial unders, partial sub-muscular):
The objective of placing the implant under the “pectoralis major muscle” is to provide better implant coverage on the outer nad inner top area only, that is to say to create a better cleavage.  Again this type of placement may not be the best suited for every patient and you should thoroughly discuss this subject with your Plastic Surgeon. This option might benefit those patients with little breast tissue that want to have a more natural look, instead of a rounded, pronounced cleavage line without rippling to the upper inner and upper outer regions of the breast.
This type of surgery is a little more intrusive.  The surgery involves separating the pectoralis major from the pectoralis minor and chest wall and placing the implant between these two.  Post-op experiences are also harder with this type of placement.  The feeling of tightness once you wake up from the anesthesia, and that of constriction and soreness are more acute than with sub-glandular placement.
  
Full Sub-muscular (Complete unders):
This placement involves the act of separating the two pectoralis, the major and minor and chest wall , as well as separating the fascia, or thick muscle covering, from the muscle itself (the rectus abdominus).  However, for ease of wording and for terminology recognition we will refer to it as full sub-muscular placement.
With placement of the implant “behind the muscle” as it is commonly referred to, the breasts may appear high and tight and even somewhat square looking for some time but patience is a virtue! Patients with this placement report that it is worth the long run.
With this placement the chance of capsular contraction is reportedly much lower and the breasts take on a more natural slope on the top as well as no visible rippling on the sides and breast lobe, or lower pole (underneath, bottom portion of the breast), that may be otherwise present in the sub-pectoral placement.