This is commonly used for relational key information and in data integrity situations. If someone could type different information into your key field, it can ruin your how relationship. Perhaps it will help to think what Yoko Ono did to the Beatles. You also have the ability to have these setting over all or some of the repetitions of a repeating field via a checkbox. Repeating fields are for the most part a legacy function from FileMaker 2.X days.
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