My OMA( not Orange) says this about discretion-"The use of commander’s discretion is considered exceptional and where possible should be avoided at home base and/or company hubs.
The decision of the commander is considered as sacrosanct and ABC Airlines has adopted a non-punitive approach in dealing with discretionary events"
It is not a rostering tool and should not be used to cover up failings in the crew establishment or poor rostering. If through no fault of your own there is an incident whilst exercising it them the regulator will look at the pilot as to why he felt able to exercise it. Any supporting paperwork or telephone call from a mamager exerting pressure would obviously attract their interest and in my opinion should be subject to a disciplinary process from the AOC holder. Discretion belongs to the Captain, having consulted his crew , not some greasypole climber in an office.