In TortoiseGit->Rebase window has these two selectors :
What is the purpose of Onto
selector, while in Upstream
you already select the branch you are rebasing onto.
The onto option can be used to rebase some commits that are between the branch and the upsteam onto some other branch.
Onto in the Git docs: https://git-scm.com/docs/git-rebase#Documentation/git-rebase.txt---ontoltnewbasegt