Set M cover letter

Hello guys, i am about to apply for set (m) next month. I have some things which i want to explain to the caseworker. Can i add a covering letter? If so, where can i add one? Do i have to upload it on ukvcas? Or is it possible to use any section to explain some other circumstances related to the financial documents? Nothing is missing from the docs just a slight change since my last spouse visa leave which i believe should be addressd to caseworker in order to avoid any delays.

If there is an Additional Information section towards the end of the online application use that, otherwise upload a cover letter with the other documents you are uploading.