The Sex Orientation Scale (SOS) was Harry Benjamin's attempt to classify and understand various forms and subtypes of transvestism and transsexualism in people assigned male at birth, published in 1966.
Benjamin feared legal consequences for surgeons who performed sex reassignment surgery, and focused on the patients being able to pass and unlikely to regret their decision when deciding whether to recommend someone for an operation—in addition to possessing an unchanging gender identity.
[6] Sex and Gender Role Disorientation and Indecision (Males) Benjamin noted, "It must be emphasized again that the remaining six types are not and never can be sharply separated.
"[1] Benjamin added a caveat: It has been the intention here to point out the possibility of several conceptions and classifications of the transvestitic and the transsexual phenomenon.
[8] Sexual orientation is no longer regarded a criterion for diagnosis, or for distinction between transsexuality, transvestism and other forms of gender variant behavior and expression.