I'm not sure how much help this will be because I'm a little shaky, and to be honest I can't remember too much from last year. Regardless, I'll give it a crack, but take this answer with a grain of salt. I'm sure others will correct parts of it.
For Section B, I used sub-headings. I'm not sure whether there is a requirement one way or the other. In general, my first paragraph was under the sub-heading "Context", which outlined factors such as the apparent relationship between interlocutors, location of the correspondence, potential audience, and anything else which would affect the way that the conversation was performed.
The subsequent 3-4 paragraphs were under headings either consistent with the five sub-systems, or things such as "Turn-taking and topic management". This really depends on the prompt with which you are provided.
Lastly, I would conclude by trying to link all of the sub-headings together to relate back to the context of the situation. It is also important to link the context of the correspondence to each sub-heading, commenting on how the contextual factors shaped your prompt.
Apologies if that didn't make sense or isn't much help!