Hello @likhitasura
Navigate to All > Service Portal > Widgets
Name is Standard Ticket Conversations
You can notice the server script is first checking which all journal fields are available and writable. If both( work_notes and comments), then server script chooses first primary writable field as primary journal field (which is comments oob) and work_notes as secondary journal field.
So when HTML (post button is clicked), then client knows which journal field to use based upon the above:
