People asked me if they should enable it or not. I had no answer for them as I did not know what it means or does.
Could not find a discussion or ticket; rPHEX9f897dbcb9cbebea42ad716633da3bcf83e315de provides no reasons.
Then found https://www.mediawiki.org/wiki/Phabricator/Help#Search_terms where it is documented.
After enabling "Expert Mode" on https://phabricator.wikimedia.org/settings/panel/developer/ , the advanced Search interface at https://phabricator.wikimedia.org/maniphest/query/advanced/ exposes five additional fields: `Parent IDs`, `Subtask IDs`, `IDs`, `Columns`, `PHIDs` (see screenshots below).
Setting is used in two code places:
* https://phabricator.wikimedia.org/source/phabricator/browse/wmf%252Fstable/src/applications/search/field/PhabricatorPHIDsSearchField.php per custom rPHAB3ed56bdd557d600631cc32e678e7503338ebe45e and custom rPHABb3a61693ced41e1763110eab47f820d2ea5cd72c
* https://phabricator.wikimedia.org/source/phabricator/browse/wmf%252Fstable/src/applications/search/field/PhabricatorIDsSearchField.php per custom rPHAB63c5e293cf0d806d2387ee1acd69f47a4ebeb99b
```
[acko@fedora phabricator]$ grep -r "expert-mode" .
./extensions/src/expert/ExpertModeSetting.php: const SETTINGKEY = 'developer.expert-mode';
./phabricator/src/applications/search/field/PhabricatorIDsSearchField.php: ->getUserSetting('developer.expert-mode');
./phabricator/src/applications/search/field/PhabricatorPHIDsSearchField.php: ->getUserSetting('developer.expert-mode');
./deployment/libext/misc/src/expert/ExpertModeSetting.php: const SETTINGKEY = 'developer.expert-mode';
./deployment/phabricator/src/applications/search/field/PhabricatorIDsSearchField.php: ->getUserSetting('developer.expert-mode');
./deployment/phabricator/src/applications/search/field/PhabricatorPHIDsSearchField.php: ->getUserSetting('developer.expert-mode');
[acko@fedora phabricator]$ grep -r "ExpertMode" .
./extensions/__phutil_library_map__.php: 'PhabricatorExpertModeSetting' => 'src/expert/ExpertModeSetting.php',
./extensions/__phutil_library_map__.php: 'PhabricatorExpertModeSetting' => 'PhabricatorSelectSetting',
./deployment/translations/projects/phabricator_ext/core/qqq.json: "0697a4a521e77f96": "Used in:\n\n[https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable//src/expert/ExpertModeSetting.php$33 ExpertModeSetting.php:33]",
./deployment/translations/projects/phabricator_ext/core/qqq.json: "9818855a7a1ea556": "Used in:\n\n[https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable//src/expert/ExpertModeSetting.php$24 ExpertModeSetting.php:24]",
./deployment/translations/projects/phabricator_ext/core/qqq.json: "aa5fe7b0dc66e1b8": "Used in:\n\n[https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable//src/expert/ExpertModeSetting.php$34 ExpertModeSetting.php:34]",
./deployment/translations/projects/phabricator_ext/core/qqq.json: "e20bdc892a74ee50": "Used in:\n\n[https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable//src/expert/ExpertModeSetting.php$12 ExpertModeSetting.php:12]",
grep: ./deployment/.git/modules/libext/misc/index: binary file matches
./deployment/libext/misc/src/expert/ExpertModeSetting.php:final class PhabricatorExpertModeSetting
./deployment/libext/misc/__phutil_library_map__.php: 'PhabricatorExpertModeSetting' => 'src/expert/ExpertModeSetting.php',
./deployment/libext/misc/__phutil_library_map__.php: 'PhabricatorExpertModeSetting' => 'PhabricatorSelectSetting',
```
{F41507329}
{F41507331}