I have guides on my forum and they use the [guide] prefix.
Suggestion: Guide threads are assigned a prefix that we can select in acp, this way when creating a guide thread the only prefix available to the user is the guide prefix (or perhaps allow a few to be selected for those that have different prefixes for guides, tutorials, DIYs, etc).
So in acp something like:
Prefixes available to users for guide threads:
[select from list]
Default prefix:
[select one]
This is already available, as the add-on adds a new forum type where you can select the available prefixes and the default prefix

However, if you want this for a General discussion forum with mixed thread types, I would need to add additional options for that