The Key Definition window shows the fields that make up the keys used to sort, store, and retrieve records in each table. Each field in a key is referred to as a key segment. Each table can have multiple keys, and each key can have multiple segments.
For example, a customer table typically would have two keys defined for it. One key could contain the Customer Name field, because this is the field a user would most likely use to search for a record. However, a customer name may not be unique. For instance, there may be more than one “John Smith”. To avoid this problem, each customer could be assigned a unique customer ID. A second key composed of the Customer ID field could be added to the table. This key would uniquely identify customers, even if they have the same name.
The Key Segments list displays the list of fields that will be used as segments in the key. Records will be sorted primarily according to the first key segment. Records that are identical for the first key segment will be sorted again by the second key segment. For example, if the Key Segments list contains two segments: [last_name, first_name], in that order, records will be sorted primarily by the last name, and records with the same last name entry would be sorted again by the first name.