Femaleness is a very complex, multi-faceted concept. There are many ways to identify as femaleness and these can vary from person to person. For instance, there are those who consider themselves femaleness (like me), but are also trans men and those who identify as more of a hybrid, a little of both.
The term "femaleness" has been a misnomer for a long time. The concept of femaleness is multifaceted, but I think the first step to getting closer to understanding the concept is to understand how we are categorized, and how those labels have become outdated. The first step is to understand that a trans woman is a person who has had sex with both men and women, and that she identifies as both female and male. She identifies as a female and a male. Some people call her femaleness, but we also call her female-to-male, male-to-female, and trans-to-male. The term "female-to-male" is a modern, outdated, and inaccurate usage that is not based on any actual scientific or medical data. Femaleness is something a person is born with, not something someone chooses. There is no medical evidence that identifies any one gender as superior or inferior. The word "female" is a biological designation and does not have any special legal significance. It is simply a biological designation. To say that gender is socially constructed is false.