A table holds records of a particular type. A field defines one part of each record. The dictionary stores field definitions, including type and reference target. A schema map shows tables and their relationships, which helps you decide where a record belongs and how to join its references.
Table extension and a reference are different relationships. Incident extends Task and inherits common task fields. Caller is a reference to User; that relationship does not make every incident a user. A sys_id identifies a record, while a display value helps people recognize it. Similar display names can point to different records. Before building a report or import, inspect the table, inherited fields, reference targets, and the grain you expect one record to represent.