I need help in fully comprehending the sentence below. Please take a look and thanks in advance. :)
"Various typologies of signs have been proposed, but three fundamental classes of signs seem to stand out as requiring different approaches: the icon, the index, and the sign proper."
What does 'as requiring different approaches' mean? I can understand the sentence only up to the point where it says 'but three..stand out'.
What is requiring what?
Can I replace 'as requiring' to 'which requires'? If not, how can I say this confusing part differently?
Is as + v-ing certain usage of 'as'?