When doing research into something it's often a good idea to get a proper definition of the topic. I found a good one which has some decent flexibility within it and it came from Portraiture by Shearer West. I'm not reading the whole book, but the first chapter (p21-41) gives a good indication about what you can expect to be judged on when making a portrait. According to Shearer West a portrait depicting a particular person with particular attention to their face and body. Their likeness is a requisite as well as an indication of the type of person that they are (eg. their social role). Next on the list is a distinction of the Body and Soul or subjectivity of a person which is something that depends on cultural beliefs at the time about what forms identity, psychology and character. These are combined with an engagement in the process of the sitting, to convey a relationship between the artist and the sitter at the time of the sitting. These are dimensional, that is, West's definition provides some flexibility allowing for cultural beliefs and fashions.
In Chapter 2 (p43-69) she discusses the role of portraiture as a work of art, as a biography, as a document, proxy (staying connected to the subject) or gift, commemoration and memorial, and as the portrait as a political tool.
West, Shaearer. Portraiture. From the Oxford History of Art series of Books by Oxford University Press, UK. 2004