"Discretionary spending" is a misnomer. Except for the pay of certain few federal officials (federal judges, and perhaps a handful of others), which cannot be decreased during their time in office, it's all discretionary. It's in the Constitution; you can look it up.
When people speak of "non-discretionary" spending, they're referring to the spending done to keep promises of previous generations of politicians, spending which is normally done on auto-pilot. The default is to keep those promises, no matter how outmoded, unwise, unconstitutional or even mathematically impossible.
Everything else is "discretionary spending".
Speaking of all federal spending, not so-called "discretionary spending", is a more useful number, in terms of the future of the republic.
For flinging partisan mud at politicians, past and present, not so much.