Clarification about dataSet format

Hello

as we know, the invoke service response send back to the app the payload with or without data.

In our case, we are receiving the payload with data and its dataSet is similar to the following (depending on the fields configured in the app: we configured only the customer id)

The question is: the rows array contains the configured field received by the program (the customer id) but we also receive the data you can see in the first column which format seems to be something like:

RIID_;some id;unix timestamp

the question is: what does the first column of the dataSet represent? I guess is needed to uniquely relate the records in the dataSet we'll form when we'll send it back to the application via the