Q: What does the status 'Proofing decision letter started' mean?

Some journals, to suit their specific workflows, like to split their status updates into mini-status updates, or smaller status updates, so to speak. If it were only ‘Decision letter started,’ it would not have been entirely clear whether it’s an accept or a reject decision. But as it includes ‘proofing’ (after submitting the revised manuscript), in all probability, it means an accept decision. :-) This is because the proofing stage, which is about checking the finalized version of the document before it is published, happens once an article is accepted for publication. However, you should wait for the final decision letter from the journal to confirm the acceptance.

Until then, to prepare yourself for the proofing stage, you may go through these resources:

• What is the minimum number of comments to be included while proofing my paper?
• Can I correct an error in citation at the page proof stage?
• Can I replace an image at the proofing stage before publication?

Hope that helps. And congrats on the acceptance! Or mini-congrats, in this case. :-)