Edges are presented one-byone and the annotator has to choose a label for it (note that there is a vague label in case the TempRel is not clear or does not exist).
in practice, every annotation comes at a cost, either time or the expenses paid to annotators, and as more edges are annotated, the marginal "benefit" of one edge is going down (an extreme case is that an edge is of no value if it can be inferred from existing edges).