always corresponds to the PAGES field, while proceedings is always associated with BOOKTITLE or JOURNAL), or as certain global regulations about complete structures (e.g., at least one word should be tagged as verb when performing a sentence-level POS tagging).
in the figure, for example, the segments C 1 and C 3 are identified as two argument segments (which are mentions of types T 1 and T 3 respectively) and are mapped to two "nodes", and the segment C 2 is identified as a gap segment that connects the two arguments A 1 and A 3 .