What choice is granted by “in shāʾa farraqahu wa-in shāʾa tābaʿahu”?

Kitab al-Sawm: Illness, Travel, Make-up Fasts and Delayed Qaḍāʾ

• Waiving make-up or paying fidyah
• Separating or consecutively performing make-up
• Changing an obligation into voluntary worship
• Fasting only in Ramadan