@ used to describe communication
意思疎通を描写するために使われる(usedが過去分詞だから形容詞としての解説だね)

in which

A the situation and the speakers' relationships,
その場の状況や話者動詞の関係性が(ここまでS')

B as well as other things such as a speaker's voice and movements,
(他の事物、例えば話者の声や動作だけでなく)

C are very important
とても重要であるような時に(=in whichにつながる)(V'C')

D in understanding the meaning of what is said,
発言されたモノの意味。意図を理解する際に

E rather than just the words that are used:
(どんな単語が使われたか、ではなく)