They serve different purposes. "Reply to Thread" is used to quote multiple posts: you select posts with the
Multi-Quote this message button (The last link from the right below each post), after you finish selection, click "Reply to Thread" to add all quoted posts into the input box.
"Reply to Thread" actually doesn't suggest Submit or Post.
If you mis-used the button and lost your draft, try this:
https://www.accessforums.net/showthread.php?t=23400
The exiting post should be there, you may want to clear browser cache or try a different browser.