Man pages matching name rt::extension::customfieldsonupdate in section 3pm:

RT::Extension::CustomFieldsOnUpdate.3pm.en	debian 10 / rt4-extension-customfieldsonupdate
RT::Extension::CustomFieldsOnUpdate.3pm.en	debian 11 / rt4-extension-customfieldsonupdate
RT::Extension::CustomFieldsOnUpdate.3pm.en	debian 12 / rt4-extension-customfieldsonupdate