Windows Diff for Exchange Customizations.
The customizations in Exchange are manual edits to config files that are not exposed to the gui or ECP.
They are overwritten upon patching/upgrading to newer Cumulative Updates or Security Updates (CU's or SU's).
Be sure to save a copy of the configs in an immutable directory that are clearly marked with the date.
To inspect differences after and patch/upgrade, use the DIFF command.
diff (cat 'E:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\ecp\Web.config') (cat e:\atemp\overrides\V15\ClientAccess\ecp\Web.config-v221013)
This will also work over UNC paths:
diff (cat 'E:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\ecp\Web.config') (cat \\server-name-here\e$\directory-name-here\overrides\V15\ClientAccess\ecp\Web.config-v221013)