It is used for marking entity boundaries. The relation type information is obtained from a predefined set of relations and the relation role information is represented by the numbers 1 and 2.

University of Michigan - Ann Arbor

This tagging scheme reduces the joint extraction task to a tagging problem. Intuitively, a single tag is assigned to a word that encodes entity as well as relation label information, automatically leading to joint inference. 

Joint Label for Joint Inference

Techniques for Jointly Extracting Entities and Relations: A Survey

1. the word position in the entity
2. the relation type
3. the relation role (argument number)

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