I think that for “parent” it means the comment you are clicking, eg.:

|-- comment A

|---- subcomment B

if you have that enabled and you click on A, both A and B are collapsed

if you have that disabled and you click on A, only B is collapsed

due to my username, I am morally obliged to comment :)

ten years of sync and I never noticed that, thanks!

posting with joy from Sync!