Question about record.id

I've been reading about the unique record ID associated with each record in the Knack database. But, I 'm new to Javascript, and it's not clear to me in the API examples how some fields are populated using record.id.

For example, in Knack's Attendance Manager app, the variable attendance_date = record.id.

Could someone shed some light on how attendance_date is populated with the correct date using record.id.

I thought the record.id was a unique identifier for a record row.

Thank you.