فَلَا يَجُوزُ صَوْمُهُ إِلَّا بِنِيَّةٍ مِنَ اللَّيْلِ.
Translation: Such fasting is not valid without intention made at night.
This is the decisive rule for make-up Ramadan fasts, unspecified vows, and expiatory fasts: intention must already have been made during the preceding night. A first intention formed in the morning does not fulfil the specified obligation.
The distinction concerns designation. Ramadan has an externally assigned date; an outstanding duty needs identification through intention.