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The follows the syntax from the Select label from [Insert link here] MM5.3.0 8 Label Configuration. Each of the options are the “id” of the sort field in the search. For example DigizuiteSystem_Framework_Search comes out of box with the following sort fields:
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This means that if you add or remove a sort field, you need to update the label correspondingly. In addition the missing option will be logged in the F12 console as a yellow warning.