I'm surprised by the confusion this is causing. The difference is simply that some spending is legally obligated contracts to third parties and can't be unilaterally cancelled by the government, while other spending can be unilaterally cancelled. If you can just change your mind and not spend (say you delay buying a car for a year), that's not a "default".
Expansion, with three broad categories:
1. Discretionary spending. By definition; at the government's discretion. It is passed annually in a budget and can be (and often is) changed mid-year....usually increases, but it can also be decreased. Simply with a few strokes of a pen.
2. "Mandatory" spending. This is for "entitlements". The word "mandatory" is a reference to the fact that this spending doesn't have to be approved annually, but rather is long-term programs that automatically do their thing unless changes are made. Unless changes are made. So the same legality applies to these as to discretionary spending: The government can change or cancel these programs whenever it wants.
3. Debt obligations. These are legally binding financial contracts (loans) with 3rd parties that the government can't unilaterally cancel: you can delay buying a car (#1) but you can't delay payment on a car you already own. The people who own the loan have to agree to delay the payment.
Failure to pay #3 is a "default". Failure to fulfill a financial contract. #1 and #2 are the government simply changing its own spending intentions and making different choices. They can't be a "default" because they aren't a legally binding contract to anyone but themselves.
Failure to pay #3 is a disaster. Our debt supports our currency. Failure to pay interest on the debt immediately impacts the value of our currency, similar to the way a bank failure wipes out bank account value.
Failure to pay #1 and #2 is not a disaster. They would reduce the GDP in the same way cancelling any spending reduces the GDP. There is economic "harm", but no currency collapse.
Ultimately a label is just a label, but when someone says "a default would be a disaster", the variation in impact between one interpretation and the other is vast. By the looser definition, we've already defaulted and the "disaster" is so minor that few people are even aware it happened.