ediff-setup-diff-regions-function is a variable defined in `ediff-diff.el'.
Its value is

  • Automatically becomes permanently buffer-local when set.
  • This variable may be risky if used as a file-local variable.

value is a function symbol depending on the kind of job is to be done.
For 2-way jobs and for ediff-merge, it should be `ediff-setup-diff-regions'.
For jobs requiring diff3, it should be `ediff-setup-diff-regions3'.

The function should take three mandatory arguments, file-A, file-B, and
file-C. It may ignore file C for diff2 jobs. It should also take
one optional arguments, diff-number to refine.