One of the key tasks in natural language processing is that of Information Extraction (IE), which is traditionally divided into three subproblems: coreference resolution, named entity recognition, and relation extraction.
iE corpora are typically annotated with information corresponding to these subtasks (MUC (Grishman, 1995), ACE (NiST, 2000)), facilitating the development of systems that target only one or a subset of the three problems.