It cannot be a multipart field.
It must contain unique values.
It cannot contain null values.
Its value cannot cause a breach of the organization's security or privacy rules.
Its value is not optional in whole or in part.
It comprises a minimum number of fields necessary to define uniqueness.
Its values must uniquely and exclusively identify each record in the table.
Its value must exclusively identify the value of each field within a given record.
Its value can be modified only in rare or extreme cases.