How does adding a Topic affect my post's reach?
Adding a Topic increases your post's visibility — it does not limit it. Your post remains fully discoverable to everyone. Selecting a Topic also places it in that topic's feed, so people browsing that category can find it too.
How to add a Topic:
Create a new post.
On the New post page, find the Topic button.
Select the topic that best fits your content.
Your post will appear in the general feed and in the topic feed you selected.