Regarding Part B of GST DRC-01A (which is the Statement of Facts / Explanation required for submitting a reply to a GST demand notice):
Part B is basically your detailed explanation or submission in response to the notice.
Currently, DRC-01A (Part B) cannot be submitted online on the GST portal.
You are required to prepare the response offline, sign it, and submit it physically or via email to the jurisdictional GST officer (as specified in the notice).
Some states may allow submission via email or an offline portal, but the official GST portal doesn't have a direct facility to upload Part B.
Make sure to keep an acknowledgement or proof of submission for your records.