The Lower Dan Tien is often referred to just by the name Dan Tien by many Tai Chi and Chi Kung practitioners and as hara or Tanden by many karate practitioners.
Dan Tien translates as Field of Elixer, and it is generally thought that it refers to a pool of Chi energy. I personally consider them to be energy gates and I have now seen where others seem to be doing the same.
The lower Dan Tien is generally considered to be the bodies physical and energetic center. A basic ready reserve of Chi is stored in this area but it is mistakenly thought by many that the lower Dan Tien holds all of the bodies store of Chi. This has been taken so far that you can readily find a lot of overweight martial arts masters who talk sometimes jokingly and unfortunately sometimes seriously about their chi belly.
The true nature of the lower Dan Tien is the same as the other Dan Tiens in the body. The Dan Tiens are gateways for body energy to pass from one area of the body to another and from one aspect of the body to another such as from the energy in the body into the physical or from energy into spirit. Having said this my next statement will prove most controversial but is not said lightly or without much research.
Dan Tiens are doors or gateways. As a result of that fact the lower Dan Tien has a storage reservoir for Chi that is readily available for use. But the lower Dan Tien is not the storage area for all of the extra chi in the body unless the practitioner does not have much chi and / or is getting near to death. I say this because for the lower Dan Tien to hold most/all of the reserve chi in your body means that most of your chi has been used up.
So, where is the Chi in the body actually stored?
The answer is: In the bone marrow.
Where is the largest source of bone marrow in the body?
The answer is: In the upper legs and hips. This is the reason for dropping the energy down through the body to settle and store it.
I have had a quite a few masters tell me this behind closed doors and this seems to be closed door / indoor student knowledge. From many years of practice and comparison and contrast I have come to believe from experience that this is where the chi in the body is actually stored.