WinSCP overwrites the existing b.txt.
It's a synchronization, not a backup.

Thanks Martin. Just to further clarify, see this scenario:

- First full backup, copies all files:
a.txt, b.txt, c.txt

- second backup, changed file b.txt, synchronises file b.txt. But b.txt exists on target. How will WINSCP work? Create a versioned file b-001.txt ?? Or ?

ReeX wrote:

Thanks Martin, I have read the guide however I couldn't understand how to perform a full backup and differential copies

It always does "differential copies". That's what synchronization is about.
If the target/backup folder is empty, the "differential copy" will of course do a "full backup".

Thanks Martin, I have read the guide however I couldn't understand how to perform a full backup and differential copies

That's what synchronize command does:
https://winscp.net/eng/docs/scriptcommand_synchronize

Basically an incremental backup. You first make a full backup and then you backup only what has changed since first full backup

No, just changed files, so that it could be done a initial full backup and then only what has changed

Do you mean to transfer only changed parts of files?

Can WinSCP create differential sync?