I remember that when I entered Kpop there were Dancers, Vocals and Rappers which you could be on Main, Lead or Sub and then there were the "special positions". Leader, Oldest and Maknae are obvious, but Center, Visual and Face of the Group were always special.
I know that some groups treated visual like a Main/Lead thing (even though I luckily never saw a sub visual, that would have been painful).
Center is something that is still explainable, but Face of the Group which was *not* always the most popular member caused some confusion back then.
I haven't seen the term Face of the Group being used these days, so maybe it isn't in fashion anymore?