I don't know if there is a definite answer to this question. I prefaced the writing sample by including a cover page where I stated the name of the work, the date of completion, the nature of the excerpt (e.g. pages x-y of chapter z), and I also provided 3-5 sentences describing the thesis from which I took the excerpt. With regards to the excerpt itself, I didn't truncate it. I simply looked for a coherent argument. I picked up where the argument began and ended where it ended.
I think they are trying to form an idea of your writing style and structure as well as your research approach and how you incorporate scholarly sources into your work. However, I encourage you to include at least one sample that that reads from beginning to end. My advisor who used to teach at Oxford said this is very important.