Correct me but there seems to be a free vs. paid plan? Seems free vs. paid is about reserving/custom domain vs. random one and a lot of security related featur

Correct me but there seems to be a free vs. paid plan? Seems free vs. paid is about reserving/custom domain vs. random one and a lot of security related feature. But the basic tunneling works for free am I correct in that? Also what does the user really mean in terms of ngrok concurrent user or kind of named seat.