The phrase "matter of discretion" means that a decision or action is left to the judgment or choice of a person or authority, based on their own preferences or opinions. Full definition
Health care insurance is a subject matter of discretion that is not to be taken blindly. (policybazaar.com)
The regulator decided the nature of the slot was «a matter of discretion» for director general Mark Thompson and other BBC executives and that a blanket ruling was not suitable. (politics.co.uk)