Simple Post Inserter takes various inputs through the settings page and determines three things:
The content you wish to display in your posts, could be plain text, could be some HTML code. By default the admin of the site is responsible for any of the content submitted.
The location where the content should be displayed within the post.
Post restrictions. By default if no other setting but the content and location are declared, the plugin will insert the content in all of the posts available on the site. This may not be what you desire, but you can filter by post type, category, and ID.
Currently the only parameter for placement inside the posts is based off of total paragraphs in a post. This will likely change in a future update to something a little more sensible such as: lines of text or character counts.