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Currently a tag key can only have one string field as the value. Since it is only this one text field, a user cannot list multiple different strings in an array, but you can append to the string if needed. For example, if it is "CostCode" as the Key, the value is "123". To add value to this, you would need to have a value like "123,234".
Reference: https://learn.microsoft.com/en-us/answers/questions/693896/how-to-add-multiple-values-to-one-tag
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