When enabled on reseller level and enabled on client level, you edit domain settings in 2 places.
Domain specific settings: /client/phpini.php
Here you can choose to show/hide errors and choose what errors to report.
(Switch to reseller of the specific client)
Client specific settings: /reseller/domain_edit.php?edit_id={CLIENT_ID}
Here you can enable/disable PHP as a whole, enable/disable PHP editor.
Here you can change some PHP settings, for example: can user change settings for: allow_url_fopen , display_errors, disable_functions, can user use php mail.
But also execution, input time, memory limit, post max size and upload filesize.
Hope this answers your question!