Fat storage - the yellow bone marrow acts as a storage reserve of fatty acids.

Yellow bone marrow makes up the majority of bone marrow stroma, in addition to smaller concentrations of stromal cells located in the red bone marrow.

Since flat bones are usually thinner than the long bones, they only have red bone marrow, rather than both red and yellow bone marrow (yellow bone marrow being made up of mostly fat).

The medullary cavity (medulla, innermost part) is the central cavity of bone shafts where red bone marrow and/or yellow bone marrow (adipose tissue) is stored; hence, the medullary cavity is also known as the marrow cavity.