ediff-skip-merge-regions-that-differ-from-default

ediff-skip-merge-regions-that-differ-from-default is a variable defined in `ediff-merg.el'.
Its value is

nil

• Automatically becomes buffer-local when set.

Documentation:
If t, show only the regions that have not been changed by the user.
A region is considered to have been changed if it is different from the current
default (`default-A', `default-B', `combined') and it hasn't been marked as
`prefer-A' or `prefer-B'.
A region is considered to have been changed also when it is marked as
as `prefer-A', but is different from the corresponding difference region in
Buffer A or if it is marked as `prefer-B' and is different from the region in
Buffer B.

You can customize this variable.