Two terms that are used interchangeably, but what do they actually mean?
Disposable money is the money you receive from your pay after taxes and deductions/contributions. This is your net income, or your take-home pay.
Discretionary money is the money you have left after using your take-home pay and applying it toward all your fixed expenses (debt, utilities, insurance premiums, mortgage, etc…) This is the money you have more substantial control over – you decide how much to spend on food, entertainment, and so on.
If you pay yourself first, that could be counted in your “expenses” as a bill to yourself!