egit replacing single file - HEAD Revision vs File in git index

Question

In the egit git eclipse plugin, I know I can "reset" a single file by right clicking on it and selecting Replace With > HEAD Revision. What is the difference between replacing with HEAD revision vs File in git index?

Answer 1

HEAD is the version of the file that has already been committed.

The index is the version of the file that has been staged (added) in preparation for a commit, but not yet committed.