This site uses tags and tags can be created by users with Trust Level 3. (Users in group - Trust Level 3)
Two new tags discussion and on-topic-only have been created.
To help in administering certain topics so that if they go off topic, it can be politely noted that the topic is tagged on-topic-only.
Adding a tag is simple, when you create the topic just select the tag from the list provided, start typing the tag, or create a tag (TL3). Also the current limit is 5 tags per topic, but that can be changed.
To see all the tags, just click the hamburger 
in the upper right and select tags:
Also this topic was tagged discussion
while Steadfastness of a remote answer collector, Web Prolog style was tagged on-topic-only
How they appear on the main page.
I know it is confusing to see: Split Topic on-topic-only
Suggestions for improvement welcome.
Adding or changing tags
If you need to add or change tag(s) after the post is created.
- Open the topic
- At the top of the topic will be the title with a pencil after the name
Click on the pencil
There is now an input selector for adding, selecting one or more tags.
- Select a tag
- When done, click the check mark to accept the current state of the information.
For more details about using tags in Discourse see: It’s Time We Talked About Tags