The following paragraphs are Dr. Simeons words from the manuscript: 

In the human body we can distinguish three kinds of fat 

STRUCTURAL FAT.  Structural fat fills the gaps between various organs, a sort of packing material. Structural fat also performs such important functions as bedding the kidneys in soft elastic tissue, protecting the coronary arteries and keeping the skin smooth and taut. It also provides the springy cushion of hard fat under the bones of the feet, without which we would be unable to walk.

NORMAL FAT RESERVES. The second type of fat is a normal reserve of fuel upon which the body can freely draw when the nutritional income from the intestinal tract is insufficient to meet the demand. Such normal reserves are localized all over the body. Fat is a substance which packs the highest caloric value into the smallest space so that normal reserves of fuel for muscular activity and the maintenance of body temperature can be most economically stored in this form. Both these types of fat, structural and reserve, are normal, and even if the body stocks them to capacity this can never be called obesity.

ABNORMAL FAT.  But there is a third type of fat which is entirely abnormal. It is the accumulation of such fat, and of such fat only, from which the overweight person suffers. This abnormal fat is also a potential reserve of fuel, but unlike the normal reserves it is not available to the body in a nutritional emergency. It is, so to speak, locked away in a fixed deposit and is not kept in a current account, as are the normal reserves.

When an obese person tries to reduce by starving themselves, he will first lose his normal fat reserves. When these are exhausted he begins to burn up structural fat, and only as a last resort will the body yield its abnormal fat reserves, though by that time the person usually feels so weak and hungry that the diet is abandoned. 

It is just for this reason that obese people complain that when they diet they lose the wrong fat. They feel famished and tired and their face becomes drawn and haggard, but their belly, hips, thighs and upper arms show little improvement. The fat they have come to detest stays on and the fat they need to cover their bones gets less and less. Their skin wrinkles and they look old and miserable. And that is one of the most frustrating and depressing experiences a human being can have.