The higher fusion, where the female is incorporated in the male, is regarded as the structure of the male, which is nearly the structural height of Keter. The lower fusion, in which the male is incorporated in the female, is regarded as the structure of the female, which is the structural height of Ḥokhma alone. However, since the structure of the male does not contain opacity (ovyut) in and of itself but only as a result of its incorporation within the female, and although that is enough for the aforementioned emergence of the ten sefirot from below to above called head, nevertheless this structure cannot expand downward from above to below to form the level of the body, meaning the enclothing of the lights in the vessels.
This is because fusion through collision against the opacity in the partition that results only from the incorporation of the female within the male is insufficient for the expansion of the vessels to form the body of a partzuf since the measure of opacity is insufficient. Therefore, the structure of the male contains only a head without a body, and the body of the partzuf is formed only from the structure of the female, which has its own opacity.
For this reason, we name only the structure of the female by the name of the partzuf, that is, the name of the partzuf called Ab, since the main part of the partzuf is its body, which is the enclothing of the lights in the vessels, and this body emerges only from the structure of the female, as explained. Therefore, the name of the partzuf is attributed to it.