If neither "Smart Threading" nor "Compact Conversation View" are enabled you get the default layout for replies. I'm calling this "Flow Chat" because it's a bit like a flow chart for the conversation that, unlike simple thread lines or indenting, shows the relationship between the replies in detail:

Again, direct replies to the OP are separated by thick dashed lines, so you can see replies beneath them form separate conversations, with those replies separated by thin dashed lines. But the big difference are the bracket-thread lines. Second level are gray. Third level are blue. Fourth level are orange. Fifth level are green. Sixth level are red. Seventh+ level are purple. Friendica never indents anything more than thread_level_7.

If you compare the second conversation here to the "smart threaded" version you can see how this shows the actual relationship and depth of the replies, where "smart threading" does not.