For myself, it is normally both. A currently active example - I split 9 different posts from “How Carney’s doing, Canada?” P&E thread on battery speculation to create a new topic: Next Generation of EV Batteries by BYD and others - Real, Speculative, or Vaporware?.
First, the entire side track began by talking about Carney allowing a number of Chinese EV’s into the market. Starting with the post I first split (which, as you said, made MacDoc an unintentional OP) were all about EV battery tech and especially Donut’s take. None of them referred back to the actual topic of the thread. So I didn’t want it to renew the hijack, but the posts were a good discussion in their own right. But doing a split, especially where I have to filter a mess of on-topic and hijacks from each other, is a good bit of work, ends up with clumsy titles (like the one I created), and sometimes upset inadvertent OP’s.
Which is why I, and almost all the other mods suggest you create your own OP so you have better titles and control. And on several occasions, I’ve offered to move old posts out of the thread to the new one (generally not to a Pit thread though).