There may be slight differences as you’re using the PC version and I’m on Android.
I thought the Trust Levels and Badges? was a really good example of when to Reply as a Linked Topic. The conversation had started in Uploaded Pictures Thread (part 2), but was clearly not about uploading pictures any more, so I split off a new Topic in reply to your message about Trust Levels and Badges. 
The conversation continued, but the topic didn’t go off-topic.
If you click on the timestamp in the top right of every message you can get the post id and an address you can use.
I’m sure they’ll do a user guide at some point.
And ETA is ‘Edited to add’.