As the first large-scale annotated corpus that follows the lexically grounded, predicate-argument approach in D-LTAG (Lexicalized Tree Adjoining Grammar for Discourse) (Webber, 2004), the PDTB regards a connective as the predicate of a discourse relation which takes exactly two text spans as its arguments.
the text span that the connective is syntactically attached to is called Arg2, and the other is called Arg1.