Restful API - Update Records Question

Hello, in the documentation for the RESTful API, it states "Update can use the key field on the table, or any other supported unique field". Does anyone know what the "other supported unique field" parameters would be?

Instead of using the key field, I would like to use a different unique field I have called "ShiftId", to update existing records. When utilizing this field it does not update based on that field. It just errors with; "Adding non-unique value for field with ID \"69\"."

I cannot find any documentation to figure out what would make it a supported unique field.

Any help is greatly appreciated.

Thank you!