"B" and "I" represent the position of the word in a trigger to solve the problem that a trigger word contains multiple words such as "take over", "go off" and so on.
the total number of tags is N t = 2 ⇤ |N eventT ype | + 1, where |N eventT ype | is the size of the predefined event types and |N eventT ype | = 33 in this paper as stated above.