Seth Godin's smallest viable audience seems like an interesting idea to adopt.
Most often, we want our work to reach a wider audience. So we tend to cover all our bases when creating stuff.
When we define the smallest viable audience for our work, we can become better at producing something useful and something the audience can connect to. Being very specific allows us to create with the knowledge that there is someone that our work is tailored to. Think of it as creating bespoke content.
Applied to writing, we can choose who we want to write for. It could be someone you know or don't know. It could even be a note to your selves (future, present, or past).
So who is your smallest viable audience?
Addendum: I always had an audience taxonomy available on this site for a few years. Though I haven't explicitly made it visible to readers, it has always been there. You can see an example here: bibliophiles for.