I found some old messages about this topic, but I think it’s worth a clarification:
if I move a file within a shared folder, then syncthing will just broadcast a “rename” command
if I copy a file to another shared folder, then syncthing will transmit a new file, but the remote machine basically already has all the blocks, so there’s very little data transfer.
if I move a file to another shared folder, then there’s no guarantee: depending on the folder rescan order, possibly the file is transmitted again.
These still seem valid. Also, #1 is not guaranteed, as it’s not broadcast as a rename, its broadcast as an addition and a delete, the remote side translates that into a rename. Now messages of files changing are batched, and if they happen to fall into different batches (because there are more files changing in some way forcing the two messages to be split between batches) the rename would not be noticed and might end up a copy or a redownload.
I think “redownload” for #1 should be rare, while “becomes copy plus (later) delete” is probably quite common as deletes are sent last and there might be other changes that get sent in between as Audrius says.