Answer:
the subject of your document
Step-by-step explanation:
The reason this is not a key topic is because you have already picked the subject of your topic, this makes up the purpose which can be changed, but the subject is something to be written about, something to be described. The purpose of your document would only be changed when you would want to put yourself in the audiences shoes and want your purpose to be known. This is where the audience of your document comes in, if you do not grasp your audiences attention or get your purpose of the document and writing forecasting your message, then you would have less of an interested audience, making this a key topic. You would also want the length of your document to be "just right" as too long of an document will simply make the audience bored, you want to make just at the right length, only relevant information is needed. This is why the subject of your document is not considered a key topic as it is the thing you're writing about, not what you're writing "through".