Question

I found only that:

// in repo, in index, content diff => changed
// in index, in workdir, content differs => modified

But I can’t understand the difference?

Answer 1

look at the javadoc for the getters in class IndexDiff:

/**
 * @return list of files changed from tree to index
 */
public Set<String> getChanged()

these are the files which differ between the tree HEAD points at (the tip of your current branch in most cases) and the Git index

/**
 * @return list of files modified on disk relative to the index
 */
public Set<String> getModified()

these are the files which differ between the files on disk (aka working tree) and the Git index