The JoinByTransformation allows us to concatenate values with a separator, e.g., join the n-grams from the N-Gram transformation. 

If you get a string back that you want to use for tagging, you should join the values with a comma. 

PRO TIP:

If you get various values and want to join them differently, you should do this in the assign value action.

Variables: FirstName, LastName, Email
Desired Outcome: FirstName LastName (Email)

Join By Comma: FirstName, LastName, Email

Assign Value Action: “$FirstName$ $LastName$ ($Email$)“

Specific definitions

Join all IPTC Subjects by comma

"transformations": [
    {
        "kind": "JoinByTransformation",
        "separator": ",",
        "traceRefId": null
    }
],
    "storeIn": "allIptcSubjects",
    "traceRefId": null